Generalized Møller—Plesset perturbation theory applied to general MCSCF reference wave functions

We report the application of a generalized Møller—Plesset perturbation theory to multiconfigurational reference wave functions. The theory makes use of a more compact first-order space than earlier investigations and results in a computationally simplified approach. The low-order perturbation theory applied to multiconfigurational reference functions obtains a significant fraction of dynamic correlation energy for a relatively low computational cost. The importance of using a reference wave function that dissociates properly is discussed. Results are reported for the symmetric dissociation of H2O and the energy difference between the 1A1 and 3B1 states of CH2.