Person:

Sherrill, C. David

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https://hdl.handle.net/1853/71694

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Organizational Unit

School of Computational Science and Engineering

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Now showing 1 - 10 of 16

Oscillations in meta-generalized-gradient approximation potential energy surfaces for dispersion-bound complexes

(Georgia Institute of Technology, 2009-07) Johnson, Erin R. ; Becke, Axel D. ; Sherrill, C. David ; DiLabio, Gino A.

Meta-generalized-gradient approximations (meta-GGAs) in density-functional theory are exchange-correlation functionals whose integrands depend on local density, density gradient, and also the kinetic-energy density. It has been pointed out by Johnson et al. [Chem. Phys. Lett. 394, 334 (2004) ] that meta-GGA potential energy curves in dispersion-bound complexes are susceptible to spurious oscillations unless very large integration grids are used. This grid sensitivity originates from the saddle-point region of the density near the intermonomer midpoint. Various dimensionless ratios involving the kinetic-energy density, found in typical meta-GGAs, may be ill-behaved in this region. Grid sensitivity thus arises if the midpoint region is sampled by too sparse a grid. For most meta-GGAs, standard grids do not suffice. Care must be taken to avoid this problem when using, or constructing, meta-GGAs.
Career theoretical studies of bond-breaking, diradicals and non-dynamical correlation

(Georgia Institute of Technology, 2009-06-22) Sherrill, C. David
Improvement of the coupled-cluster singles and doubles method via scaling same- and opposite-spin components of the double excitation correlation energy

(Georgia Institute of Technology, 2008-03-28) Takatani, Tait ; Hohenstein, Edward G. ; Sherrill, C. David

There has been much interest in cost-free improvements to second-order Moller-Plesset perturbation theory (MP2) via scaling the same- and opposite-spin components of the correlation energy (spin-component scaled MP2). By scaling the same- and opposite-spin components of the double excitation correlation energy from the coupled-cluster of single and double excitations (CCSD) method, similar improvements can be achieved. Optimized for a set of 48 reaction energies, scaling factors were determined to be 1.13 and 1.27 for the same- and opposite-spin components, respectively. Preliminary results suggest that the spin-component scaled CCSD (SCS-CCSD) method will outperform all MP2 type methods considered for describing intermolecular interactions. Potential energy curves computed with the SCS-CCSD method for the sandwich benzene dimer and methane dimer reproduce the benchmark CCSD(T) potential curves with errors of only a few hundredths of 1 kcal mol⁻¹ for the minima. The performance of the SCS-CCSD method suggests that it is a reliable, lower cost alternative to the CCSD(T) method.
Benchmark full configuration interaction and equation-of-motion coupled-cluster model with single and double substitutions for ionized systems results for prototypical charge transfer systems: Noncovalent ionized dimers

(Georgia Institute of Technology, 2007-10) Pieniazek, Piotr A. ; Arnstein, Stephen A. ; Bradforth, Stephen E. ; Krylova, Anna I. ; Sherrill, C. David

Benchmark full configuration interaction and equation-of-motion coupled-cluster model with single and double substitutions for ionized systems EOM-IP-CCSD results are presented for prototypical charge transfer species. EOM-IP-CCSD describes these doublet systems based on the closed-shell reference and thus avoids the doublet instability problem. The studied quantities are associated with the quality of the potential energy surface PES along the charge transfer coordinate and distribution of the charge between fragments. It is found that EOM-IP-CCSD is capable of describing accurately both the charge-localized and charge-delocalized systems, yielding accurate charge distributions and energies. This is in stark contrast with the methods based on the open-shell reference, which overlocalize the charge and produce a PES cusp when the fragments are indistinguishable.
Hybrid correlation models based on active-space partitioning: Seeking accurate O(N ⁵) ab initio methods for bond breaking

(Georgia Institute of Technology, 2006-08) Bochevarov, Arteum D. ; Temelso, Berhane ; Sherrill, C. David

Møller-Plesset second-order (MP2) perturbation theory remains the least expensive standard ab initio method that includes electron correlation, scaling as O(N ⁵) with the number of molecular orbitals N. Unfortunately, when restricted Hartree-Fock orbitals are employed, the potential energy curves calculated with this method are of little use at large interatomic separations because of the divergent behavior of MP2 in these regions. In our previous study [J. Chem. Phys. 122, 234110 (2005)] we combined the MP2 method with the singles and doubles coupled cluster (CCSD) method to produce a hybrid method that retains the computational scaling of MP2 and improves dramatically the shape of the MP2 curves. In this work we expand the hybrid methodology to several other schemes. We investigate a new, improved MP2-CCSD method as well as a few other O(N ⁵) methods related to the Epstein-Nesbet pair correlation theory. Nonparallelity errors across the dissociation curve as well as several spectroscopic constants are computed for BH, HF, H₂O, CH+, CH₄, and Li₂ molecules with the 6-31G* basis set and compared with the corresponding full configuration interaction results. We show that among the O(N ⁵) methods considered, our new hybrid MP2-CCSD method is the most accurate and significantly outperforms MP2 not only at large interatomic separations, but also near equilibrium geometries.
The electronic structure of oxo-Mn(salen): Single-reference and multireference approaches

(Georgia Institute of Technology, 2006-04) Sears, John S. ; Sherrill, C. David

Using single- and multireference approaches we have examined many of the low-lying electronic states of oxo-Mn(salen), several of which have not been explored previously. Large complete-active-space self-consistent-field (CASSCF) computations have been performed in pursuit of an accurate ordering for the lowest several electronic states. Basis set and relativistic effects have also been considered. For the geometry considered, our best results indicate the ground spin state to be a closed-shell singlet, followed by a pair of low-lying triplet states, with additional singlet states and the lowest quintet state lying significantly higher in energy. Hartree-Fock and density functional theory (DFT) results are obtained and are compared to the more robust CASSCF results. The Hartree-Fock results are qualitatively incorrect for the relative energies of the states considered. Popular density functionals such as BP86 and B3LYP are superior to Hartree-Fock for this problem, but they give inconsistent answers regarding the ordering of the lowest singlet and triplet states and they greatly underestimate the singlet-quintet gap. We obtained multiple Hartree-Fock and DFT solutions within a given spin multiplicity, and these solutions have been subjected to wave function stability analysis.
Hybrid correlation models based on active-space partitioning: Correcting second-order Møller-Plesset perturbation theory for bond-breaking reactions

(Georgia Institute of Technology, 2005-06) Bochevarov, Arteum D. ; Sherrill, C. David

Møller–Plesset second-order (MP2) perturbation theory breaks down at molecular geometries which are far away from equilibrium. We decompose the MP2 energy into contributions from different orbital subspaces and show that the divergent behavior of the MP2 energy comes from the excitations located within a small (or sometimes even the minimal) active space. The divergent behavior of the MP2 energy at large interfragment distances may be corrected by replacing a small number of terms by their more robust counterparts from coupled-cluster (CCSD) theory. We investigated several schemes of such a substitution, and we find that a coupling between the active-space CCSD and the remaining MP2 amplitudes is necessary to obtain the best results. This naturally leads us to an approach which has previously been examined in the context of cost-saving approximations to CCSD for equilibrium properties by Nooijen [ J. Chem. Phys. 111, 10815 (1999) ]. The hybrid MP2–CCSD approach, which has the same formal scaling as conventional MP2 theory, provides potential curves with a correct shape for bond-breaking reactions of BH, CH₄, and HF. The error of the MP2–CCSD method (measured against full configuration-interaction data) is smaller than that of MP2 at all interfragment separations and is qualitatively similar to that of full CCSD.
The X-1 Sigma(+)(g), B-1 Delta(g), and B ` (1)Sigma(+)(g) states of C-2: A comparison of renormalized coupled-cluster and multireference methods with full configuration interaction benchmarks

(Georgia Institute of Technology, 2005-03) Sherrill, C. David ; Piecuch, Piotr

Unusual bonding and electronic near degeneracies make the lowest-lying singlet states of the C2 molecule particularly challenging for electronic structure theory. Here we compare two alternative approaches to modeling bond-breaking reactions and excited states: sophisticated multireference configuration interaction and multireference perturbation theory methods, and a more “black box,” single-reference approach, the completely renormalized coupled-cluster method. These approximate methods are assessed in light of their ability to reproduce the full configuration interaction potential energy curves for the X , B , and B′ states of C2, which are numerically exact solutions of the electronic Schrödinger equation within the space spanned by a 6-31G* basis set. Both the multireference methods and the completely renormalized coupled-cluster approach provide dramatic improvements over the standard single-reference methods. The multireference methods are nearly as reliable for this challenging test case as for simpler reactions which break only single bonds. The completely renormalized coupled-cluster approach has difficulty for large internuclear separations R in this case, but over the wide range of R = 1.0–2.0 Å, it compares favorably with the more complicated multireference methods
High accuracy ab initio studies of Li-6(+), Li-6(-), and three isomers of Li-6

(Georgia Institute of Technology, 2005-02) Temelso, Berhane ; Sherrill, C. David

The structures and energetics of Li6+, Li6− and three isomers of Li6 are investigated using the coupled-cluster singles, doubles and perturbative triples [CCSD(T)] method with valence and core-valence correlation consistent basis sets of double- to quadruple-ζ quality (cc-pVXZ and cc-pCVXZ, where X = D−Q). These results are compared with qualitatively different predictions by less reliable methods. Our results conclusively show that the D4h isomer is the global minimum structure for Li6. It is energetically favored over the C5v and D3h structures by about 5.1 and 7.1 kcal mol−1, respectively, after the inclusion of the zero-point vibrational energy (ZPVE) correction. Our most accurate total atomization energies are 123.2, 117.6, and 115.7 kcal mol−1 for the D4h, C5v, and D3h isomers, respectively. Comparison of experimental optical absorption spectra with our computed electronic spectra also indicate that the D4h isomer is indeed the most stable structure. The cation, anion, and some higher spin states are investigated using the less expensive cc-pCVDZ basis set. Adiabatic ionization energies and electron affinities are reported and compared with experimental values. Predictions of molecular properties are found to be sensitive to the basis set used and to the treatment of electron correlation.
Full configuration interaction potential energy curves for the X-1 Sigma(+)(g), B-1 Delta(g), and B-'1 Sigma(+)(g) states of C-2: A challenge for approximate methods

(Georgia Institute of Technology, 2004-11) Abrams, Micah L. ; Sherrill, C. David

The C₂ molecule exhibits unusual bonding and several low-lying excited electronic states, making the prediction of its potential energy curves a challenging test for quantum chemical methods. We report full configuration interaction results for the X 1Σg+, B 1Δg, and B′ 1Σg+ states of C₂, which exactly solve the electronic Schrödinger equation within the space spanned by a 6-31G∗ basis set. Within the D2h subgroup used by most electronic structure programs, these states all have the same symmetry (1Ag), and all three states become energetically close for interatomic distances beyond 1.5 Å. The quality of several single-reference ab initio methods is assessed by comparison to the benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree–Fock reference exhibits large nonparallelity errors (>20 kcal mol−1) for the ground state. The excited states are not accurately modeled by any commonly used single-reference method, nor by configuration interaction including full quadruple substitutions. The present benchmarks will be helpful in assessing theoretical methods designed to break bonds in ground and excited electronic states.