Abstract
The automatic choice of active spaces and initial orbitals for multiconfiguration self-consistent-field (MCSCF) calculations on organic molecules and transition states is discussed. The use of unrestricted Hartree-Fock natural orbitals (UHF-NO), restricted open-shell Hartree-Fock molecular orbitals, and natural orbitals from limited configuration-interaction computations for starting MCSCF calculations is examined. Particular examples are the ring-opening reactions of dioxetene, oxetene, dithiete and thiete and the electronic structures of the phenoxy and thiophenoxy radicals. The UHF-NO scheme was found to be particularly appropriate for the automatic definition of active spaces provided that the appropriate UHF solution can be obtained. Some difficulties with multiple UHF solutions are illustrated.
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