COMPARISON AND ASSESSMENT OF DIFFERENT FORMS OF OPEN-SHELL PERTURBATION-THEORY

Existing methods for the calculation of the correlation energy of open shell atoms and molecules using Moller-Plesset perturbation theory are discussed with special emphasis on recent advances in methods based on the restricted Hartree-Fock wave function. The convergence of the Moller-Plesset series is examined to high order for a number of small systems and it is shown that some of the methods offer significant advantage over traditional unrestricted Moller-Plesset theory. The optimized bond length and vibrational frequencies of CN, NO, and O2 are also compared for second and fourth order Moller-Plesset theory, and again it is found that the same methods give much better results than th corresponding unrestricted results. Further supporting calculations are presented on the electron affinity of CN and the dissociation energy of HCN. The new methods involve relatively minor adaptation of existing unrestricted Moller-Plesset (UMP) codes, and in the light of their superior performance are recommended instead of UMP theory for open shell molecules.