3RD-ORDER COMPLETE ACTIVE SPACE SELF-CONSISTENT-FIELD BASED GENERALIZED VANVLECK PERTURBATION-THEORY

A state-selective generalized Van Vleck perturbation theory (GVVPT), correct through third order. for the determination of the electronic structure of polyatomic molecules is presented. The method adapts Kirtman's GVVPT for use with complete active space self-consistent field (CASSCF), and orthogonal complement, zeroth-order many-electron functions. The tenability of the method is demonstrated by calculations on several well-known ''benchmark'' molecules: the Be atom, using a (12s5pld)/[7s3pld] basis set, double zeta (DZ) H2O; 6-31 G NH2; the singlet-triplet and singlet-singlet energy separations of DZP CH2; the excited states of DZP CH2+; and the dissociation of DZP + F2.