Open-shell Moller-Plesset perturbation theory based on the asymptotic method of obtaining SCF orbitals

Asymptotic method of obtaining restricted open-shell wavefunctions satisfying the correct variational conditions has been extended to excited states. Perturbation theory calculations based on this methodology have been carried out for the correlation energy in the NH2 (B-2(1)) molecule and the vertical excitation energy in BeF (B-2 Sigma(+) --> X-2 Sigma(+)). Our results for NH2 are closely related to known RMP results. Unlike spin-restricted UHF methods however, the proposed scheme deals as do the OPT1, OPT2 and ZAPT theories with the identical spatial parts for the alpha and beta orbitals, which has computational advantages. The results for the excitation energy are in agreement with experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.