Multiconfiguration selfhyphen;consistenthyphen;field study of the importance of triply and quadruply excited electronic configurations in the water molecule

摘要

The importance of triple excitations, configurations differing by three electrons from the Hartreendash;Fock reference configuration, is of considerable interest in electronic structure theory. A simple double zeta basis set O(9s5p/4s2p), H(4s/2s) has been used to address this problem for the water molecule. Configuration interaction (CI) and multiconfiguration selfhyphen;consistenthyphen;field (MCSCF) wave functions including up to all single, double, triple, and quadruple (SDTQ) excitations (a total of 17thinsp;6781A1configurations) have been obtained for this purpose. An interesting result is that the MCSCF wave function including only single excitations yields 52.3percnt; of the comparable correlation energy obtained with all single and double excitations. Using canonical SCF orbitals, triple excitations are found to contribute only 0.8percnt; of the correlation energy. However, the MCSCF procedure increases this correlation energy fraction by more than a factor of 5. The MCSCF energies for the wave functions including SD and SDTQ excitations are less than 0.001 hartree lower than the comparable straight CI energies.