Multireference-state Rayleigh--Schroedinger perturbation theory applied to the electronic states X /sup 1/. sigma. /sup +//sub g/ and EF /sup 1/. sigma. /sup +//sub g/ of H/sub 2/
The effect of the reference space on the convergence of Rayleigh--Schroedinger perturbation series within the molecular-orbitals framework is studied for the ground X /sup 1/..sigma../sup +//sub g/ and excited EF /sup 1/..sigma../sup +//sub g/ electronic states of H/sub 2/ over a wide range of internuclear separations. Near the ground-state equilibrium distance of 1.4 bohr each state is well described by a single spin-adapted configuration function. This no longer holds for the EF /sup 1/..sigma../sup +//sub g/ state around 3.0 bohr because of forbidden curve crossing. In general, the quasidegeneracy increases with the internuclear distance due to improper dissociation of molecular orbitals. A rigorous approach to define a proper reference space is discussed. It is based on analysis of convergence with emphasis on identifying intruder states. A reference space of nine spin-adapted functions is adequate in the range 1.4--8.0 bohr; giving third-order results within less than 1 x 10/sup -3/ hartree from the basis limit. Other findings are: (i) Epstein--Nesbet breakup of the Hamiltonian usually gives faster convergence as compared with the Moller--Plesset scheme. (ii) Pade approximants improve the results but only when the reference space is capable of describing the state. When this is not the case the Pade sequence is erratic and physically meaningless. (iii) With suitably defined reference space there is only a marginal difference in the results between different breakups of the Hamiltonian, and little improvement is gained by employing Pade approximants.
- Research Organization:
- Chemical Physics Department, Weizmann Institute of Science, 76100 Rehovot, Israel
- OSTI ID:
- 6101849
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 84:8
- Country of Publication:
- United States
- Language:
- English
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