THEORY AND APPLICATIONS OF SPIN-RESTRICTED OPEN-SHELL MOLLER-PLESSET THEORY

Formulae in terms of integrals and orbital energies are presented for the evaluation of the second-, third- and fourth-order energies in ope n-shell spin-restricted Moller-Plesset (RMP) perturbation theory. The performance of RMP theory is evaluated by calculating equilibrium stru ctures and harmonic vibrational frequencies of a large number of open- shell systems and by calculating energies of reactions which have been shown to be poorly described by spin-unrestricted Moller-Plesset (UMP ) theory. RMP theory removes the spin contamination present in the unr estricted Hartree-Fock (UHF) reference of UMP theory. In cases where t he spin contamination gives rise to large errors in UMP calculations, such as for the harmonic frequencies of NO, CN, CO+ and FO2 and the en ergies of reactions involving CN and FO2, RMP theory is shown to offer a substantial improvement. In cases where the Moller-Plesset series e xhibits large oscillations, such as for O2, O2+ and F2+, RMP theory is shown to offer no improvement over UMP theory; in these cases good re sults are achieved with higher-order methods such as quadratic configu ration interaction and coupled-cluster theory.