Open-shell restricted Hartree-Fock perturbation theory: Some considerations and comparisons

Timothy J. Lee, Alistair P. Rendell, Kenneth G. Dyall, Dylan Jayatilaka

Research output: Contribution to journalArticlepeer-review

87 Citations (Scopus)


A comparative study is presented of the various recently developed open-shell perturbation theories that are based on a restricted Hartree-Fock reference wave function. Included in this study are issues concerning spin contamination, implementational considerations, and numerical comparisons at the second-order of perturbation theory for equilibrium geometries, vibrational frequencies, and singlet-triplet energy differences. Based on all of these considerations, it is concluded that the z-averaged perturbation theory (ZAPT) method is to be preferred over the other recently devised spin-orbital perturbation theories, while the spin-free OPT2 method possesses some advantages and disadvantages relative to the ZAPT method. In particular, it is shown that OPT2 energies are not invariant to rotations among singly-occupied degenerate molecular orbitals.

Original languageEnglish
Pages (from-to)7400-7409
Number of pages10
JournalThe Journal of Chemical Physics
Issue number10
Publication statusPublished - 15 May 1994
Externally publishedYes


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