PubMed Health⌕ Search

Biomedical subjects

Lyudmila V Slipchenko

Publications and source records attributed to Lyudmila V Slipchenko.

6 recordsLinked to original sources
PubMed2007

Electrostatic energy in the effective fragment potential method: theory and application to benzene dimer.

Evaluation of the electrostatic energy within the effective fragment potential (EFP) method is presented. The performance of two variants of the distributed multipole analysis (DMA) together with two different models for estimating the charge penetration energies was studied using six homonuclear dimers. The importance of damping the higher order multipole terms, i.e. charge dipole, was also investigated. Damping corrections recover more than 70% of the charge penetration energy in all dimers, whereas higher order damping introduces only minor improvement. Electrostatic energies calculated by the numerical DMA are less accurate than those calculated by the analytic DMA. Analysis of bonding in the benzene dimer shows that EFP with inclusion of the electrostatic damping term performs very well compared to the high-level coupled cluster singles, doubles, and perturbative triples method. The largest error of 0.4 kcal/mol occurs for the sandwich dimer configuration. This error is about half the size of the corresponding error in second order perturbation theory. Thus, EFP in the current implementation is an accurate and computationally inexpensive method for calculating interaction energies in weakly bonded molecular complexes.

Algorithms↗
PubMed2006

Advances in methods and algorithms in a modern quantum chemistry program package.

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

Algorithms↗
PubMed2006

Efficient strategies for accurate calculations of electronic excitation and ionization energies: theory and application to the dehydro-m-xylylene anion.

In the dehydro-m-xylylene (DMX) anion [Munsch; et al. J. Org. Chem. 2004, 69, 5735], three nearly degenerate orbitals host four electrons, which results in a large number of nearly-degenerate electronic states. By using this challenging example, we assess the performance of the multireference "brute force" approach and the two-step schemes based on single-reference methods for calculating accurate energy differences. Different schemes for calculating adiabatic ionization potential (IP) of DMX- are also investigated. IP calculated by single-reference based schemes is in excellent agreement with experiment.

Anions↗
PubMed2005

Spin-conserving and spin-flipping equation-of-motion coupled-cluster method with triple excitations.

We report the implementation of the spin-conserving and spin-flipping variants of the equation-of-motion (EOM) coupled-cluster (CC) model, which includes single and double excitations in the CC part and single, double, and triple excitations in the EOM part, i.e., EOM-CC(2,3) [Hirata, Nooijen, Bartlett, Chem. Phys. Lett. 326, 255 (2000)] for closed- and open-shell references. Inclusion of triples significantly improves the accuracy of EOM-CCSD for excitation energies (EOM-EE-CCSD) and its spin-flip (SF) counterpart, EOM-SF-CCSD, especially when the reference wave function is strongly spin-contaminated. A less computationally demanding active space variant with semi-internal triples has also been implemented. The capabilities of full and active space EOM-CC(2,3) are demonstrated by applications to CO(+) and CH radicals as well as to the methylene and trimethylenemethane diradicals and the dehydro-m-xylylene triradical.

Journal Article↗
PubMed2004

Reactivity and structure of the 5-dehydro-m-xylylene anion.

The electronic structure of dehydro-m-xylylene anion (DMX-) has been investigated by using chemical reactivity studies and electronic structure calculations. DMX- has been generated in the gas phase via the sequential reaction of trimethyl-3,5-bis(trimethylsilylmethyl)phenylsilane with F- and two molecules of F2. Reactivity and thermochemical properties of the ion indicate a phenyl-like anion (1a), consistent with theoretical predictions. Density functional calculations predict a nonplanar triplet anion, with an allenic singlet anion slightly higher in energy. The driving force for the out-of-plane distortion is more efficient charge delocalization that is achieved at lower symmetry.

Journal Article↗