Improving CISD calculations by geminal-type reference states

The possibility of using geminal-type wavefunctions as multi-configurationa l reference states for configuration interaction (CI) calculations is inves tigated. The reference function class considered here includes 2-electron G AS, certain types of MC-SCF and APSG (antisymmetrized product of strongly o rthogonal geminals) as special cases. The method is not variational but exa ct for two electrons. The relation between the developed formalism and GVB- CI, internally contracted multi-reference CI (IC-MRCI) and EOM-CC technique s is discussed. We present applications at the CISD level. The results show that total energies generally fall between standard CISD and CCSD values. The proposed modification of CISD decreases the size-consistency error and eliminates it if the molecule is fragmented into two-electron pieces. Preli minary calculations show an appreciable improvement of CISD dissociation en ergies. (C) 1999 Elsevier Science B.V. All rights reserved.