2ND-ORDER MOLLER-PLESSET PERTURBATION-THEORY FOR MOLECULAR DIRAC-HARTREE-FOCK WAVE-FUNCTIONS - THEORY FOR UP TO 2 OPEN-SHELL ELECTRONS

Moller-Plesset perturbation theory is developed to second order for a selection of Kramers-restricted Dirac-Hartree-Fock closed- and open-sh ell reference wavefunctions. The open-shell wavefunctions considered a re limited to those with no more than two electrons in open shells, bu t include the case of a two-configuration SCF reference. Denominator s hifts are included in the style of the OPT2 method of Murray and David son, but with a modification which avoids the orbital dependence of th e original formulation. An implementation which uses unordered integra ls with labels is described, and application to the ground state of th e O2 molecule is presented.