Second-order quasi-degenerate perturbation theory with quasi-complete active space self-consistent field reference functions

A quasi-degenerate perturbation theory (QDPT) is presented that is based on quasi-complete active space self-consistent field (QCAS-SCF) reference fun ctions. The perturbation method shown here is an extension of a previously proposed QDPT with CAS-SCF reference functions (CAS-QDPT) but is a more com pact perturbation method that can employ a much smaller reference configura tion space with the same number of active electrons and orbitals as the CAS case. A computational scheme to second-order using a diagrammatic approach is described. The second-order effective Hamiltonian consists of the contr ibution from external excitations, which involve core or/and virtual orbita ls, and internal excitations, which involve only active orbitals. The impor tance of the internal excitation contribution is emphasized. The method is tested on the potential energy curves of the LiF molecule, the Rydberg exci tation energies of furan, and the transition state barrier height of the re action, H2CO -->H-2+CO. The results are in very good agreement with the cor responding CAS-SCF reference QDPT results and available experimental data. The deviations from the CAS-QDPT values in the energy are less than 0.1 eV on the average for the excitation energies of furan and less than 1 kcal fo r the barrier height of the reaction, H2CO -->H-2+CO. The deviation from th e experimental values is 0.11 eV at most for the excitation energies, and 1 .2 kcal/mol, which is within the twice the experimental uncertainty, for th e barrier height. (C) 2001 American Institute of Physics.