Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 131, Issue 20, Pages -Publisher
AMER INST PHYSICS
DOI: 10.1063/1.3265769
Keywords
beryllium compounds; dissociation energies; excited states; ground states; lithium compounds; molecular configurations; perturbation theory; potential energy surfaces
Funding
- Hungarian National Research Fund (OTKA) [NI67702]
- Indo-Hungarian project IND [04/2006]
- Hungarian National Office for Research and Technology [OMFB-01445/2006]
- U.S. Department of Energy [DE-FG02-04ER46120]
- CSIR (New Delhi)
- Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore
- DST (New Delhi)
- EPSCoR
- Office Of The Director [0814442] Funding Source: National Science Foundation
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Three recently developed multireference perturbation theories (PTs)-generalized Van Vleck PT (GVVPT), state-specific multireference PT (SS-MRPT), and multiconfiguration PT (MCPT)-are briefly reviewed and compared numerically on representative examples, at the second order of approximations. We compute the dissociation potential curve of the LiH molecule and the BeH2 system at various geometries, both in the ground and in the first excited singlet state. Furthermore, the ethylene twisting process is studied. Both Moller-Plesset (MP) and Epstein-Nesbet partition are used for MCPT and SS-MRPT, while GVVPT uses MP partitioning. An important thrust in our comparative study is to ascertain the degree of interplay of dynamical and nondynamical correlation for both ground and excited states. The same basis set and the same set of orbitals are used in all calculations to keep artifactual differences away when comparing the results. Nonparallelity error is used as a measure of the performance of the respective theories. Significant differences among the three methods appear when an intruder state is present. Additionally, difficulties arise (a) in MCPT when the choice of a pivot determinant becomes problematic, and (b) in SS-MRPT when there are small coefficients of the model function and there is implicit division by these coefficients, which generates a potential instability of the solutions. Ways to alleviate these latter shortcomings are suggested.
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