The reduced model space method in multireference second-order perturbation theory

A technique for reducing computational effort in multireference second-order perturbation theory with very large complete active space (CAS) SCF reference functions is proposed. This is achieved by construction of an effective Hamiltonian within the space of configurations dominating the reference function expansion. The method is tested on the standard problems of singlet–triplet (1A1–3B1) separation in the CH2 radical and vertical excitation energies in formaldehyde. Numerical results show that good accuracy can be obtained even with substantially reduced model spaces.