2ND-ORDER PERTURBATION-THEORY WITH A CASSCF REFERENCE FUNCTION

Second-order perturbation theory based on a CASSCF reference state is derived and implemented. The first-order wave function includes the full space of interacting states. Expressions for the contributions to the second-order energy are obtained in terms of up to four-particle density matrices for the CASSCF reference state. The zeroth-order Hamiltonian reduces to the Møller-Plesset Hamiltonian for a closed-shell reference state. The limit of the implementation is given by the number of active orbitals, which determines the size of the density matrices. It is presently around 13 orbitals. The method is illustrated in a series of calculations on H2, H2O, CH2, and F-, and the results are compared with corresponding full CI results. © 1990 American Chemical Society.