Well. There are. Things. You. Wrote. Called. Thanks. Like We're Not No thank you. This is the point when you always thank your host for inviting you to give a collage. But I wasn't invited. Until the last time I looked the last time I gave a colloquium in this department was one thousand nine hundred eighty seven. One nine hundred eighty seven and when I'm done when I'm done today you better learn why I haven't been asked to give a colloquium since one thousand eight hundred actually was a two hour colloquium about the Woodstock of physics as a matter of fact I still have the slide somewhere. Anyway so today I'm going to talk about not physics but the history of physics and pro-drug is right that in writing that that electrodynamics book I became interested in the history and the history of physics is a fascinating subject but a part of it which hasn't really been studied by historians is the second half of the twentieth century solid state physics I don't know if they're scared off from it or they think it's all deadly dull I don't know which but it's simply untouched and so I decide perhaps it was something to do here and because this is in fact the fiftieth anniversary of the publication of a rather famous paper by Walter Cohn which at the I don't thank you I'll tell you quite a bit about over the next hour and the thing he created which is called density functional theory and how this theory came about and how this particular man came to do it is it is a pretty interesting story and so that's what I proposed through to do today so we'll have to start out who is well to CONUS is someone who isn't really well known outside of you know he's well known to the people who know him well and other people don't know who he is at all and what is density functional theory in certain parts of physics this is a this is unknown other parts it's very well known We'll talk about that and there really a question which interested me. As a nation to historian is how did Conan come to create this theory in the context of mid twentieth century cells that physics and so they'll have to understand some of the some of the context and. And how the whole thing came about so that's the that's the plan so OK so here is Walter. Not I saw him quite recently he looks sort of the same they passed a certain age you don't actually look any different but still it's still quite quite surprising So he's an American professor of physics at the University of California at Santa Barbara. But he won the Nobel Prize as you merriment know in one thousand nine hundred ninety eight and the work that he won for was done in one thousand nine hundred sixty three nine hundred sixty four in one thousand nine hundred sixty five and so this is one of these photos taken when you go into the booth right and close the the curtain at that time he used that in a application to the Guggenheim Foundation. For reasons that we'll see in a moment and so I want to I want to trace this the story now what is the end of the functional theory I'm just going to give you sort of the one slide version of this and so there's a way function with a with a knot sign through it and that's really what this is about that's the functional there is an approach to the electronic structure of matter atoms molecules solids plasmas all those things in which the an electron away function is replaced by the electron density as the fundamental variable right so we're all taught in quantum mechanics that's the way function the way function the way function and most of the time we learn about one particle wave function and then maybe in quantum mechanics two or Advanced Quantum mechanics you learn about the many particle wave function but that's what you're supposed to focus on now the trouble with this thing is that it's a function of three N. variables the position coordinates of all the electron say and that's fine maybe if you're dealing with a small atom but suppose you're arrested in a solid wood twenty ten to twenty third atoms and that's a lot of electrons and it's a lot of things. Keep track of so it's sort of difficult whereas the density. And it's just a function of X. Y. and Z. So it's a function of three variables so you can see is the size of the system gets large there are considerable advantages to working with the density if you can get away with doing it right and that's really the key to the entire business. OK now so here's sort of the advertisement for this for this subject for ordinary matter composed of electrons and nuclei let me get this water bring over here. Do you have to completely avoids the Schroedinger equation he just don't have to deal with this many electron object nevertheless it permits calculations of molecular crystal structure bond energies cohesive energies bi racial lasting properties optical thermal magnetic superconducting potential energy services chemical reaction rates all sorts of wonderful things and here I've just picked a random figure from one of the many many many papers that use this method to do a calculation and this case it was to do understand the properties of hydrated at any so. Particularly for systems with large numbers of electrons there's really no other way to proceed the methods of quantum chemistry become too awkward. This give you some indication about popular this theory is so this is number of citations versus time you'll notice that the scale here this is sixteen thousand All right this is kind of a big number and you can see now actually if you go back to ninety sixty three it took up exponentially there too but the time constants a little different and actually this this is a good exponential over this entire range too and it actually was sort of uninterrupted by the Nobel Prize in fact it started around one thousand nine hundred and I'll say some more about that a little bit later essentially this is where the chemists took over all right. So let's start the story Walter cone there. Walter you know those that spell out a little different this is the original spelling. In Vienna so his father Solomon and get to all over here Solomon I had a business. In which he produced and distributed art postcards So these are postcards which would have the Venus de Milo on them or famous paintings or famous photographs of Yanna that he commission to have taken on it was a very literate family very a typical sort of middle class Jewish borzois family in Vienna at this time in one thousand thirty two his sister Mina is right there now this is sort of an interesting time to be living in Vienna in one thousand thirty two zero here is one year later actually it's an incredible place in many ways so in every field of endeavor we have pathbreaking people are not sure Emberg in music Sigmund Freud of course the inventor of modern psychology called popper the philosopher Gustav Clint the artist just an amazing place with a great ferment of intellectual activity going on and a lot of all kinds of activity going on just in the commercial sense as well so a very very cosmopolitan place very interesting. Nine hundred thirty three is interesting but unfortunately that's also the year when Hitler comes to power in Germany and what happens few years later in March of one hundred thirty eight is an event which is called the Anschluss and this is when. Hitler and his armies simply took over Austria and when I interviewed Walter he wanted it made perfectly clear that does not mean some violent takeover he wanted to emphasize that the Austrian population was delighted to have Herr Hitler come over and take over their country all right so this is a very important thing so there we can actually see the Nazis in their cars. All right so very soon thereafter there is a if I'm. Event. In history which is actually the Germans coined this term Kristallnacht and it's usually translated as the night of the broken glass what it was was a large scale Countrywide organized orgy of violence in which synagogues and Jewish businesses were burned and destroyed Jews were killed and arrested in large numbers. The word some of you may know is given to this and it took place simultaneously in one night all over the country a cone himself his Their apartment was ransacked and when he returned home from school that day his mother was cowering in the corner of their apartment so it's it was a terrible time in general and a particularly terrible time to be a Jew. Now as it happened this. Cone was up able to escape by a remarkable vehicle namely a operation which goes today by the name of the kindertransport kinda means child and so between November nine hundred thirty eight month May nine hundred forty there was an organized effort to try to get as many Jewish children out of Nazi occupied areas as possible and they came from Vienna Prague Brylin and it was a complicated system there was a lottery system of various sorts and ten thousand unaccompanied children and so the parents could not go with them they were given allowed to have one suitcase and one bundle of that sort right and they were dead and shipped by train to Rotterdam and from there by boat over to London where they were supposed to have families already had agreed to take them into their homes right now of course not everybody actually managed to have that and they smuggled themselves onto the kindertransport anyway and made their way children you had to be sixteen or less so Cohen was at the very top of that age when he managed to. Scape another famous Kindertransport escapee was our No pansy Yes the Nobel Prize winner who'd Co-discoverer the three degree black body radiation and that is several two other Nobel laureates too it's an incredible group of people who escaped by this way but try to imagine at this age to be put on a train by yourself to go to a foreign country many of them never seeing their parents again. Now as it happens because father was in this postcard business he had clients all over the world that he shipped is various things too and one of them was a gentleman in London named Charles health who ran a sort of an art distributorship and so this is just an advertisement in the in the times of the sorts of things that he was doing but among other things he. Sold resold cones postcards and so Walter's father wrote to how you never met him just say would you be willing to take in my children and how often is his wife Eva said yes they would and so first the the daughter his sister Mina she got away sort of right away the Nazis were a little allowed female children to escape to leave and they were much more slow to let the boys leave and and actually Cohn got away on the on one of the last Kindertransport trains before they were shut down completely Now the last he managed to make it through to London with his family took him in and the main thing you had to do You had to agree that you're going to provide an education that's what the the idea with the thought was that after a couple of years everything will be over they can send the kids back home so what you were supposed to do in the ensuing two years was to provide education that was the key thing so in fact after some interesting adventures that I don't tell you about he wound up at the East Grinstead County Grammar School. All right it actually looked like this at the time when he was there and he still on ish has one of the textbooks that he used at that time and it was a brand new textbook nine hundred thirty nine right on heat and you can sort of see what's going on here it's sort of a it's a thermodynamics text and when you look at it I have to tell you it's a it's a pretty sophist it's is a high school it's a pretty sophisticated thermodynamics book I was I was impressed in fact I could be teaching from it I'm sorry to say I'm teaching permanent enemies right now and I could easily be using this particular book to teach from so it's interesting so he was getting a rather good education on the other hand he didn't know English and so the headmaster at this school cut a deal and there was a German teacher at this school and so Cohn met every day with that teacher and thirty minutes were spent with the teacher teaching cone English and the other thirty minutes were spent with cone teaching the teacher German and that's how they had how they did it so. So everything is happy for about nine months or so and then in May nine hundred forty Germany invades the low countries that's. Netherlands and Belgium and places in that general area and what this did was it sent a panic or panic as you might imagine through the folks in Britain and so the cry went up to the cry went up to in turn aliens all right and so that meant that anybody who had a passport from Germany Vienna sorry there was no more Austria so you had German passports you had Czechoslovakia passports if you had a passport for any of those places then the cry went up to through around you up and put you in term of camp and in fact there was a famous and very brief meeting of Winston Churchill soon after he took over. The prime ministership in which he said collar the lot right which meant just collect all of these foreign nationals and put them behind barbed wire in various places some of these people many of these people by the way were scholars who were places like Oxford and Cambridge and who were doing the equivalent into their Ph D.'s or postdocs or they were junior scientists or junior you know literary people or philosophy they had come from the continent and they were simply working in England all of them were scooped up at least all the men were scooped up many fewer women were and interned in these barbwire places one of the places was the Isle of Man off the coast of the of. Of of Britain where there was a huge term and camp so. After a certain period time the British were worried about German prisoners of war and all their camps of this sore or filled with these in turn ease. Of various sorts some of these people they had worry were actual German spies many many of them or not they were in fact a large number of them were Jews who would simply you know come to Britain to continue their education but they were worried that they that that actual prisoners of war would need to space so they had to do something with the attorneys they had to ship them out somewhere so they contacted the Canadian government and the Australian Government and said we would like to send you prisoners of war all right you're all part of the Commonwealth of Nations and so it's time to step up and help and so after some huffing and puffing the Canadians in The Australian said yes we'll take some of your prisoners of war and so they prepared. In terms of camps of various kinds and here they are by the way this even many Canadians until relatively recently were unaware of this little piece of their history it's sort of interesting. And in fact what happened it is called got put on. B.M.S. so by ASCII which was a Polish ship which had been captured by the Brits and then repurposed as a transport ship and so they made their way across the Atlantic and I remind you there were you boats operating at that time it was a very dangerous thing to do and in fact one of the ships that was sent full of internals was sunk and everybody was lost and as a matter of fact it was that if it vent which caused such an uproar in Britain that they discontinued the entire In term and process they were horrified their last was too late for Conan and he wound up spending time in several of these camps as a matter of fact an entire year was spent doing that the Canadians were a bit surprised when folks on loaded from the boat and they were hardened prisoners of war they weren't hard Nazis they were just a bunch of Jews. Some of them Orthodox Jews in these long black outfits and hats and they didn't look too dangerous that's because they weren't they had just been rounded up because they weren't British nationals and so they were distributed amongst amongst these camps now it turns out that that turned out to be not such a bad thing after all on the one hand they were kept within barbed wire and so they had to something one for most of the time and so most the time they were lumberjacking So at the camp called ripples that was wont So here we have ripples ripples as a camp and Farnham as a camp and so here we say let's see where the ripples there's ripples and there's Farnham and you see most of them are in this part of the country. And so they worked for twenty cents a day lumberjacking All right. On the other hand they were in tourney's themselves realize that there was a large population of of people who needed an education these were the high school age kids and so amongst the internally is a large number of Cambridge scholars and Oxford scholars a very large number of them and so they themselves got together and formed a school all right and they were. Well to revive classes and so here we go this is a chemistry class which is in C. the tiny space it's in all crowd together there's a blackboard so all the scholars wrote to people all around the world saying we're interned here they weren't incommunicado they were just being held there Canadians weren't it wasn't a labor camp or the conventional sort it wasn't a concentration camp it was just go and term and camp of the sort that this that the United States did the Japanese citizens on the West Coast so they were free to communicate they got lots of books shipped their way they got blackboards and all sorts of things and these classes you have to understand are being taught by true scholars I mean really very very good people and so as a matter of fact. Near the end of his in term a period Cohen had chance to have a chance conversation with we with the commandant of the camp and he said he wishes that his children had been able to attend the camp school because they were better than the local Canadian schools that his kids had to go to. So here's an example of two of the books that called himself got for his own self study so one of them is a famous book by Hardy a course of pure mathematics already in one hundred thirty eight in its seventh edition. And this is a really interesting one this he would have known about because there were so many Cambridge scholars they would have told him all of them had to use this for to study for the for the mathematical try post which of the exam they give a Cambridge in or if you get your degree and so he would have heard about this this book which was brand new published in one hundred thirty nine by John Slater introduction of chemical physics he told me he ordered it out of a catalogue it just sounded interesting when you look at this book now again this is one thousand nine hundred forty we're talking about Conan seventeen he's seventeen this is a serious book this is the first book which combined thermodynamics and that that's the core mechanics into one textbook nowadays that's called thermal physics everybody publishes books like this Slater was the first to do it plus the other half of the book is about the economy. Properties of matter it's a very sophisticated book and you can imagine that the young Cohn in this in term of camp spent read every page of it and every page of the hardy as well and so despite this incredible trajectory that it was on he managed to get a pretty shockingly good education. So there it is at eighteen and one hundred forty one soon after he was released from a term and not a bad looking kid all right so there he is he is he's in Canada I mean he can't really get back he's in communication he can write letters back and forth with his parents which he does until they are deported to Auschwitz and murdered which is just a couple of years later but he really had no money and no way to get back to even if he wanted to go back he couldn't. And so what happened was that he made his way to Toronto because there was a wealthy family there that made it their business to once again find young inter nees who had been released and to help them continue their education and one of these people one of these families was a gentleman who was a professor at the Banting Institute at the embassy Toronto. And he in fact made it possible for Cohen to come and live with he and his wife and to provide an introduction to this gentleman Leopold Infeld who was at that time in the department of mathematics applied mathematics at the embassy of Toronto in felt as is is actually well known. Among other things for this book which he wrote in collaboration with Einstein because he worked with Einstein for a year and actually talked him into writing this book purpose of making money for in fell for Einstein and it's actually it's a good book actually I recommend it it's worth it's worth reading. So Infeld makes through a long complicated process Infeld manages to get cone accepted to be. A student at the University of Toronto as an undergraduate already he gets essential his high school equivalency degree in the in term and camp all right so so this is this transcript from the U.S. attorney on so this is what he attended for May forty two to May forty five that's only three years so he squeezed in a four year academic career into three years now you can't read any of this but in the original the certain lack of imagination by the Toronto people it's math one math two math three math more about five math six math seven math eight and it's physics one Physics two physics three physics for physics OK so they're not so interesting there are missing things on here though now the less There's Oriental literature. Right which turns out to be when I inquired the era's you throw That's a class they still teach in which they take I don't know various classics in the Orient by the way at that time was defined as anything from Arabia east right so there'd be Persian literature in translation Arabic literature in translation Chinese literature in translation etc And that's what that course consisted of. He also had to take this is required he had to take actuary science and so this is from one of his notebooks on actuarial symbols so I guess he was a little bored and so did a little doodle of the actuary in the other world so. Physical gets good to see that certain student habits never change right so that was kind of interesting OK So he takes is a his degree there but then it's not so much the classes as the people who happen to be by an accident of timing and place at the Applied Physics class mathematics department at the university trial at that time one of them was Alexander Weinstein who was there at that time already around expert in the calculus of variations. All right so here's an example of of one of his papers he actually gave this to come to read because he wanted to recruit him to be a grad student writes a column is taking his undergraduate work there and wisely wants to grab him to be a graduate three. Other people who are his mentor is a gentleman Arthur Stevenson who took his degree from Cambridge and came to Toronto and once again it has to do with variational techniques so in this particular case a paper from one hundred thirty eight on the lower bounds there's Weinstein again but this time applied not to the classical problem of the vibration of plates which is what one thing was interested in but to a quantum mechanical problem and in fact it was Stevenson who introduced cone to the wide world of quantum mechanics and significantly the use of variational principles in order to learn the important information that you couldn't get any other way. OK So as Cohn later said in his own Nobel autobiography the variational methods in general became the first tool in my theoretical toolkit and in a very real sense they became his principal tool through the rest of his career. And has the great virtue variational methods do they're very general and very powerful. So let's see. He has a master's degree student he hung around for an extra year as he said to take a whole bunch of more physics and math classes and while I was at it he published a couple of papers so these the first two papers that he published so Coralie Journal of Applied Mathematics to spiritual gyro compass This was basically a slight variation on an on a textbook problem that he had been studying but this one is a somewhat different story so by the time. He one of the reasons that he had to graduate early was that he finally succeeded in getting into the Canadian army he actually wanted to immediately after getting out the in term and camp he wanted to serve and fight but because the army wouldn't take him eventually they go. He too and he became an instructor and they however because here's a catch twenty two because he was had been an. Austrian citizen they weren't going to send him over to fight because they were worried about his loyalty here's a guy I mean it's ridiculous so he never actually got to Europe to fight he was a drill instructor the entire time in Canada but while in the barracks he was writing this paper contour integration the theory of a spherical pendulum the heavy circle top the first reference in here is to a famous paper called the top by Felix Klein. And you can tell from reading the paper that he's absorbed an enormous amount of this paper and the mathematics in here is actually pretty sophisticated So at this time he already has absorbed quite a bit of his of math. Well it's time to go to graduate school he doesn't stay at the Rondo he originally to thinks he's going to go through the mercy of Birmingham to work with Rudolph spirals he accepts an offer from pirates and then a day later the offer comes in from Harvard and he goes to in felde and says What should I do and if it sells tell piles are going to Harvard. And so he did right and in fact in fell tells them not only should you go to Harvard but you have to work for this new hot shot that they've hired their name Julian winger and that's in fact exactly what he does so it is here arrives at Harvard one semester after Schwing arrives there. And there was an incredible class of people at Harvard at that time here are some of them these are his classmates at Harvard Phil Anderson Nico Blumberg and Roy Glauber Shaw Schlichter these are all extremely well known physicists Nobel Prize Nobel Prize Nobel Prize. Thomas Kuhn who by general acclamation was the best kid in the class. Actually became a historian of science soon thereafter and he's the one who coined he wrote a famous book called The about scientific revolution and you coined the phrase of a paradigm shift if you've ever heard the word the phrase paradigm shift used for anything it originates with Kuhn and other guy hang around with was Tom Lehrer Tom Lehrer was not a physicist but if you're of a certain age you know that he wrote musical ditties. Which were actually biting social and political commentary at the prime and it started out by him simply playing in the apartments of these various people when they got together interesting character so if you don't who Tom Lehrer is Google and listen to some of his music when you leave today so. Actually does his thesis Ph D. thesis very quickly two years two years are actually more like a year and a half but OK let's call it two years so collisions of light nuclei it's full of guess what variational principles applied to this particular problem and in fact what he developed in there is called the cone variational principle to this day trader gave him a different problem he wanted him to solve the collision of three particles. Of three particles and call. Three or one to him that he himself had tried and failed so instead he handed the Crown so here you do it right well doesn't make any progress a few years later little big day of an extremely well known Russian mathematical physics writes an enormous tome where he solves that problem anyway instead cone invents his own variational principle here the people this is these are the people that signed his thesis so there's Julian Schwing or Wendell fur it was a very well known for this is the Harvard the part I'm but this is an interesting one that's Herbert GOLDSTEIN That's the Goldstein of the classical mechanics textbook. Who was a post-doc at the time at Harvard turns out on your committee you didn't have to have just faculty you could have post-docs and so Goldstein was actually on cones. So he's trained as a nuclear physicist by shewing or with an expertise in variational things OK He then takes a post talkie hangs around working with ringer for a while another gentleman named Borowitz and they're interested in calculating the magnetic moment of the proton the neutron using meds on theory is a popular topic at that time. Unfortunately just at the same time sugar and fine men are inventing quantum electrodynamics and so right when Borowitz and Cohen are beavering away at at these magnetic moments these two incredibly seminal papers come out and clone his memoir says Boy it's and I made some minor contributions which were in far more plumbing the ultimate Depp's I soon felt almost completely useless you can imagine the boss you know is doing this incredible stuff and you're scurrying around doing nothing. So he was sort of depressed at this time. Sort of salvation came from the fact that he needed a summer job and there in Cambridge it turned out or that was they had the Polaroid Corp and he got a summer job. Because at that time photographic emulsions were just starting to be used in nuclear physics to study particle tracks and the Kodak lab had produced all of the plates that were being used and Polaroid wanted to muscle in on the business and so they hired cones this summer in tourney to explain how these tracks got made and so that's basically a solid state physics problem he did and he was a nuclear physicist so he had to learn solid state physics in order to do anything with this problem but luckily John bad luck was the chair of Harvard the time back then black and want to come to work for him but. Cohn said Now that's not easy kind of want to do that I want to work for where and so and he did but he went back to Van black and said Now I need to know some solid state physics and so then Black gave him this famous book called The modern theory of solids by Frederick sites which is published in one thousand nine hundred it's an amazing book first book on solid state physics ever written still worth reading today I have it on my shelf so for now my shelf it's on my desk. So cone absorbed solid state physics while looking for a job. He found a job at the Carnegie Institute technology he was hired by a nuclear physicist named Ed Crites who went on to have a very illustrious career at the national level in physics. This is my this is my alma mater. That's Forbes Field with the pirates used to play that's the University of Pittsburgh we used to call that the heights of ignorance So anyway now. He was going to take the job and started Carnegie Tech but then it turned out he managed to get a post-doctoral opportunity in Copenhagen at the bar Institute so there's more there is caught on and so he showed up at Carnegie Tech for one semester taught solid state physics that's why they hired him because they need a solid state physicist because sites had been a Carnegie Tech be left to go to Illinois and so they were left with no one to teach so that's why Khan was hired so he taught first semester then escaped off to Copenhagen where he discovered they didn't know it's also a physics was nobody knew what the words meant and so he is a nation so it's a physicist working at Copenhagen but the great virtue of this was that this is after the war and right when he was there Bohr put together the first enormous conference of anybody who would ever pass through the Boer Institute before the war and he asked everybody to come back and so Cohn met an enormous number of people and they formed his scientific network for the rest of his life it was a great great thing for him. Now the sources. Problems that he turned to insult a physics was to take the mathematical techniques that he knew and apply them to solve that problem so for example you have a solid A crystal is a collection of positive ions and they make this potential which I call the effects think of that as an external potential that the electrons feel right this is the so-called band structure problem and so cone developed once again variational methods to be used to study that problem and he actually using a Marchand elect mechanical calculator actually saw the show in your equation for this and he applied it to the problem of metallic lithium and this method is used to this day it's called the K K R method karoonga cone Rostock or occurring it was a. Dutch for the system had invented the same method but as he later wrote he couldn't do the calculate real calculations for a real system because the calculation one to do would have taken the entire budget for computing for his country and so he wasn't allowed to do it but Conan Rostock or did Rostock or was and was a another Toronto product who happened to be a Carnegie Tech at the same time so Conan sort of made this transition from working with streamers a nuclear physicist becoming an accidental solidstate physicist and then realizing actually I kind of like this stuff I'm going to stick with it so he actually becomes rapidly well known in the field this is a famous set of of review books was the first volume was in fifty five and it dealt with things like the one electron theory of solids That's the band structure problem what holes metals togethers they transitions semiconductors which had out of the war people began to understand the value of semiconductors and electron electron interactions and metals these were the topics right which were hot at the time and cold in fact was working in essentially all of them except for phase transitions he never really did statistical mechanics as part of his his. Work. He ran into Bill Shockley William Shockley one of the well claimed co-inventors of the transistor and that's an interesting story too but in any event at a meeting and he didn't have enough money at his job at Carnegie Tech they didn't pay very well so he was looking for another job and Shockley are he offered him a job as a matter of fact and he went back to CARNEY You know this game this is an old game so he took the offer from Bell what back to Carnegie Tech and they instantly promoted him and gave him a big raise so he stayed but he became a consultant at Bell Labs and he remained a consultant there for many summers Here's what the place looked like at the time it's kind of grim. I think but there was a. Whole set of stars there there are still Anderson who shows all his is colleague from Harvard Gregory Bonnie and Connors herring are people that's also a physicist in the room will recognize right away as making seminal contributions to that subject so he sort of imbibed all of this whenever he came for the summer time. To work the main person he met there was a Quinn lot and sure this is the lot and you're the some of you may know from the so-called wonder liquids but at that time he had colon formed a wonderful collaboration with a published a whole bunch of these papers on a whole variety of subjects which you see spanned the gamut you know from very detailed work about donor states and silicon which was important to prefer Bell Labs to ground state of a many fermion system a new mechanism for superconductivity this is just a very a year or so after the C.S.S. invented they had another idea which turned out not to be relevant for the materials that were being studied at the time but it turns out to be relevant today. OK but what's really happening in physics right around this time one hundred fifty eight is the many body problem diagrammatic perturbation theory which was developed by firemen. To study the problems of interest to him. It was quickly discovered that they could be used to study the many electron problem that is to say you have ten to twenty third electrons they all repel each other with a cool interaction they're interacting with these positive ions what the hell happens all right and so these methods turned out to be a panacea for understanding what to do here are some of the people involved in that some of these names Delmon Bruckner Goldstone maybe known to to some of you the main thing is to write down diagrams and so on how to convince himself that he could do it too and so I think it's the only paper ever were you actually write down diagrams these are these are actually human holds diagrams for those who care about it but it was a particular problem he wanted to show the classical problem we teach in elementary electromagnetism if you put a charge into a dielectric then what you get is the field of the charge screen by the dielectric constant Q. divided by capital at a constant He wanted to show that comes out quantum mechanically. Which he did. But what's interesting I like many people of his then Tige he didn't sort of become completely a name or with many body theory and diagrammatic perturbation theory and just become a technician doing that. As well see he was there said the many body problem but he didn't just sort of start doing nothing but this there are other problems of interest to him so for example analytic properties of block functions these are the running waves in a crystal image of the family surface in the vibration spectrum a metal that sounds kind of esoteric but what it is today it's known as the cone anomaly it's a type of thing one can measure with neutron diffraction and actually became quite famous for that particular piece of work which he published actually bought by himself. So we're moving here. In the one nine hundred sixty and you can sort of get this. Sense that he's doing a lot of different things and this is recognized by the American Physical Society the author Buckley prizes the highest award. Still called solid state physics in those days that assaults that a physicist could earn and there's the citation for having extended Lucy said the electron theory of matter what the hell's I mean well he had done so many different things that they didn't pick out any one thing it was a body of work he was already already at at this time sort of. A lifetime achievement award in the end even done the thing for which is really best known for which we're coming to. His life changes he's still at Carnegie tack when he gets recruited by Keith Bruckner who is a famous nuclear physicist and Roger Revelle here probably never heard of but he was the director of the Scripps Institute for Oceanography in La Jolla California and Revelle had the idea that what the University of California needs is another campus and it needs to be right here where I am we need to make the University of California at Loyola all right and he was a sort of a one man incredible force of nature who spearheaded all this thing and one of the things to do was to find stars all around the United States who were willing to come through the whole area to start this campus in a bunch of tents right and in fact Cohen was one of the ones who was recruited It was actually called the University of California you see L.J. It was actually called that for two years before changed to the University of California at San Diego which is what we know it today and so called agreed to come but the condition he put on it was that there had to become the chair all right and so that's what happened rocker becomes the chair cone comes one year goes by. And Bruckner says you know I think I'm going to take this job in Washington a think tank. And Cone finds himself holding the bag or holding the chair as the case may be. And so he wasn't happy about this but there were sort of wasn't an alternative so here's his department year later and I thank my friends at San Diego for filling in all the names of who these people were there's some interesting names here Margaret Burbidge for example who is here and John Wheatley and all sorts of interesting people some herb York well known later for Arms Control So an interesting group of people. But being a chair as our current chair can attest in our past year can attest is a very tiring thing and so. Cohen's tenure there lasted two years and as soon as he realized he was going to go he was going to be relieved of the chairmanship immediately applied the Guggenheim people and said Get me out of here I want to go to Paris and I'll work on the interaction of electrons and photons which is our quantized lattice vibrations and I want to go and recharge my batteries in Paris is one of his favorite places and so that was that picture that we saw earlier in his application so this is sixty two and he wants to spend the fall of sixty three there and he wants to do it working at the cold air mass appeal which is right here which is right I mean within spitting distance of the Luxembourg Gardens for example and other wonderful things in Paris and then his main collaborator was going to be. Who was one of the many body people who really exploited that Purcell's that physics purposes. So that so that's what he wants to do it he goes there but he has other friends who he meets there and the two most important for our purposes it one is a theorist jock for a Dell and the other an experimentalist under Reagan Yeah and they were interested in met taluk solid solutions All right so a symposium on electronic structure this happened right before Cohn got. There so he arrives and he's in this giant office where nosy as is and those years is kind of busy doesn't time to talk to him so he's wandering around trying to figure out what he should do he's supposed to work on the electrons and photons but Fredo says Here watch you take a look at the proceedings for this right and maybe there's some interesting problems that you can work on. So cone decided to think about it how do you calculate the electron energy levels and wave functions for a alloy so this is a metal alloy this is brass so it's zinc atoms and copper atoms and there you start with the zinc lattice and you randomly put copper atoms in there right and the question is how do you calculate the way functions in the energy levels and what it's a problem right because the problem the co knew how to solve everybody knew how to solve it was when you had a periodic array when the potential was perfectly periodic this problem does not have a periodic potential attack it has a random type of potential so nobody knew what to do about that and so-called thought that's a pretty good problem there were very few people thinking about the time very few so he decided to go do so. He had some notion of how to proceed at least start thinking about it because free Dellums Southwood studied the problem since these impurities these the zinc atoms could be regarded as a sort of impurities he studied the problem of putting impurity into an electron gas and actually back at Carnegie cone it had to postdocs of his Jim Langer inside Waskow study that problem so we studied if you take a background of electron gas you stick a positive ion in there we know what happens classically Well classical classical physics you can even put a charge you know into a conductor it goes right to the surface in quantum mechanically you can do this and the charge the electrons run over here. To screen this thing to cover up its charge but they have these oscillations called Pradelle oscillations and so there was some notion of how one might start thinking about a problem where you had this background of white things with these occasional blue and stuck in there maybe I could do a concentrated version of this kind of a picture and there were nuclear magnetic resonance experiments and alloys which gave some indication of this so we started thinking about that as a as a research problem. So what he says in his autobiography is that he's reading the metallurgical literature in Paris because after all alloys are what metallurgist do businesses don't work on alloys they didn't too much at that time but metallurgists of course that's their job so he says I found that the concept of an effective charge for each atom in our way roughly characterized the transfer of charge or train Tomic cells so if you have a type atom stuck in a matrix A B. you can have a charge transfer between the two so that turned out to be something you thought was important the electrostatic interaction energies are important thus in considering energetics of the system there was a natural emphasis in the literature on the specially varying electron density so we get the first notion that the electron density itself is an important variable to think about and some people you might think that that's the smoking gun for what later becomes density functional theory. Except I looked I was a spent I spent a lot time looking for these so-called metallurgical literature where this happens nothing. I couldn't find anything anywhere I ask Walter he said I don't remember I'll get back to you Well you haven't got back to me and he still doesn't remember and I'm not even convinced that exists a few years later people started thinking this way but not then the only thing the variable which appears is the atomic number of the atomic charge of a cell divided by the volume of sell some average dancer. The but the spatial variation that doesn't appear anywhere and I'm going to attribute that idea to calm himself so he starts thinking about the electron density can I make a theory of the electronic structure which depends upon the density so this is the eureka moment the question this is him writing the years later the question occurred to me whether knowledge of the density of alone determined the potential of the of our that's that what I previously called the external potential produced by the ions. So he's asking if you if I knew the exact density of the electrons could I figure out where the ions were because they make the potential which goes into the Schroedinger equation to find the total energy that's the question he asked himself. So Schrodinger we all learned you give me the potential the external potential produced by in an atom is just the potential of the nucleus in a solid it's the potential of all of nuclei and there are periodic and you saw the Schroedinger equation that means that the energy is a function now of the potential V.. The conjecture was that it's enough to know the density that'll tell me the potential and that in principle tells me the energy. The proof it turns out is by contradiction and it uses his old friend the railway variational principle and the reason was he was asking a question of such great generality that you had to have a tool which was similarly extremely general and extremely powerful and so he was led right back to the directional principle and it involves a well known inequality to anyone had who's had a quantum mechanics in kindergarten that is that the ground save energy is less than or equal to this integral website is some test wave function and of course equal if it's the exact. Around state the proof is three lines. I'm not going to the proof I'm talking about is this that the density you know that implies the potential I won't give you the three lines because they're three lines of math and that's because I can do it in one line. There is a theorem which he did not know at the time called Cato's there are well known to quantum chemists it says the exact charge density has a cost at the position of each nucleus So in other if if this is a plot this is actually of course a circus tent but these are concepts and so the idea is that that's the position of a nucleus that's the position of a nucleus that's the position of a nucleus because that's where the costs are but that's what you're trying to show that if you tell me the density as represented by these tents then I know where the ions are and if I know what the ions are I write down the potential they make and I'm done so that proves that the density implies the potential which then implies everything else there is to know about the system. That's it that's the proof so code does this by himself but it was such a remark a result in trust himself as a right is that I don't know so he looks around the office in Paris and he sees Pierre on Byrd who was a post Oct first for an O.C.R. who just arrived from Moscow by way of Harvard. And he says I got a problem for you. And homework says well I can't find those heroes anyway so they start working together so there's a theorem correct is it known all right can we establish a variational principle all right and in fact own budget a bunch of other things as well. It took them about six months to and they were fighting with various other theories saying we can prove that the density is all that matters and everybody says no that's ridiculous you need the wave function and there's no way the density is enough information you're throwing away ten of the twenty third variables and replacing it by three that's nonsense but in fact nobody could prove that they. Proof was wrong and so they write a paper this paper deals with the grounds they have an interactive electron gas an external potential it's proof that there exists a universal functional density independent of the potential such that the total energy has its minimum value that is so-called homework own theory which underlies density functional theory one hundred sixty four that's why I wrote the fiftieth anniversary of this particular result. The question arose as to what the method might be good for and call it suggest I want to try to improve current techniques for tackling band structure Holmberg whose background is in is in fluids says but best trucks are hardly complicated and that sort of thing better left to professionals young men on the cone of Conan Ross talk or write he had done bad structure back in the day you never did any more right but at least implicitly knew what to do. Nevertheless Holmberg goes and gets a job at Bell Labs and he gives a talk about his work and the people there had no enthusiasm whatsoever they didn't care about fancy theorems because they couldn't didn't help them with the many body problems that they were dealing with they had concrete specific problems and throwing a general theorem did not help so cone looked around if he could find someone else who could help turn this general result in something practical he finally goes back to Loyola where Lucia is a post-doc waiting for him and they write together three rather remarkable papers one of which is the most highly cited paper in all of physics the most highly cited paper in all physics by a very large margin self-consistent equations including exchanging correlation of facts what that does is that it combines the whole bird cone theorem with once again the really rich variational method to express exact many body theory exact in a form no more complicated than hard trees theory that. Try structure of atoms you may have heard of the Harvey Fox method that was preceded by the harshly method which was even simpler and they showed that they could reformulate this whole thing and that method it introduced a one approximation in what they could actually do calculations to produced good numerical results for real systems and that's what they did so here are the equations that said I am out of time so I'm sorry I can't tell you about them but they're really extremely straightforward right for anyone who's been involved in these sorts of things it's a very simple thing to implement and in fact numerically it's quite straightforward to do so and there was one approximation exactly one there was some functional that you had to be able to write a formula for and they use known results for the homogeneous electron gas which the many body people had been learning a huge amount about in the preceding ten years there was a great deal of known about that they simply use them wrote a paper said the first red letters our judgment is well deserved publication it's not of such urgency to warrant the speedy publication of the letter Sincerely Sam Goldsmith who is the editor so if you have a paper rejected by fizzer of letters or Science or Nature you too may yet have a paper which is the most highly cited paper in all physics if you're willing to wait fifty years or so. So here's the timeline and I'm going to conclude with this so in one hundred sixty four we have the invention of this density functional theory and this local density approximation it all comes out of an incredibly prosaic and frankly dull sounding problem how do I find the way function for an alloy right some alloy the Miller just might be interested in sixty seven Fiza start to use this thing to study the properties of crystals and surfaces nine hundred eighty international journal quantum chemist there's a misguided belief that the particle density can determine the exact grounds that chemist didn't believe it even this much after the. After the fact. But then between eighty and ninety some other chemist said you know it's not there all this proof must be right but the local density approximation is not so good. There is no perturbation theory which tells you how to systematically improve the local density approximation which stopped physicists in their tracks but a good chemist doesn't care about things like that they're willing to try anything and so they did and they tried these crazy things that no physics would ever think up and they turned out to produce results with chemical accuracy that means one K. cow from all that's a very very good. And so as a result. Eight years later Cohen gets a Nobel Prize one half of them but not in physics not in physics in chemistry and he shares it with John pople who is a card carrying quantum chemist so the one thousand nine hundred Nobel Prize was given for quantum chemistry half to a real chemist and half the Walter com for the development of density functional through today it is routinely used by physicists chemists material science biologists engineers the chairman of the Georgia Tech chemical engineering department his research is density functional calculations that's our chemical engineering department All right so very interesting this explains where this incredible rise came from once the theory in its practical applications got chemical accuracy it took off. But of course cone of self had to learn a new trade you'll see here chemistry made easy and so he's trying to learn some elementary chemistry which he you know barely ever took a system so thank you for your attention. All. Right this. Is. Before you leave. Or the other way ninety degrees next. Week and I will get something for you don't take so long for. You know know this the sister the member girls were not interned. And so she was able to stay with the British family and after the war she returned to Vienna and she went back to the business that her father had had which had been confiscated by the Nazis so she got the business back and she continued to run it until her death which was just a few years ago. Right. Right well thank you for us I hope it was a moment and I mean give me. He was. Right Cambridge chemists were mostly mathematicians that is true that was true. Yeah it was racially. Well I mean my Ph D. thesis was in density functional theory. And when I realized that it was a story that. Ought to get told by somebody. I decided should be me so. Simple as a. Right. Great thank you very much. Thank you.