ON THE CHOICE OF ACTIVE SPACE ORBITALS IN MCSCF CALCULATIONS

We describe a procedure which may be used to aid selection of the acti ve space in multiconfigurational self-consistent field (MCSCF) calcula tions for general chemical systems. Starting from a restricted Hartree -Fock calculation, we define a hierarchy of interacting virtual orbita ls for every occupied orbital. The most strongly interacting orbitals are then taken to constitute the active space in a configuration inter action (CI) calculation. The natural orbital occupation numbers obtain ed from the CI calculation are then used to choose the active space to be used in a subsequent MCSCF calculation. We illustrate our method o n a number of systems (Li-2, B-2, C-2, carbonyl oxide, and the transit ion state for oxidation of H2S by dioxirane). In all these cases, 'int uitive' active spaces are inadequate, as are active spaces derived fro m the natural orbitals of unrestricted Hartree-Fock calculations. (C) 1998 Elsevier Science B.V. All rights reserved.