On the choice of active space orbitals in MCSCF calculations

Abstract

We describe a procedure which may be used to aid selection of the active space in multiconfigurational self-consistent field (MCSCF) calculations for general chemical systems. Starting from a restricted Hartree–Fock calculation, we define a hierarchy of interacting virtual orbitals for every occupied orbital. The most strongly interacting orbitals are then taken to constitute the active space in a configuration interaction (CI) calculation. The natural orbital occupation numbers obtained from the CI calculation are then used to choose the active space to be used in a subsequent MCSCF calculation. We illustrate our method on a number of systems (Li 2, B 2, C 2, carbonyl oxide, and the transition state for oxidation of H 2S by dioxirane). In all these cases, `intuitive' active spaces are inadequate, as are active spaces derived from the natural orbitals of unrestricted Hartree–Fock calculations.