Improving CISD calculations by geminal-type reference states

The possibility of using geminal-type wavefunctions as multi-configurational reference states for configuration interaction (CI) calculations is investigated. The reference function class considered here includes 2-electron CAS, certain types of MC-SCF and APSG (antisymmetrized product of strongly orthogonal geminals) as special cases. The method is not variational but exact for two electrons. The relation between the developed formalism and GVB-CI, internally contracted multi-reference CI (IC-MRCI) and EOM-CC techniques 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. Preliminary calculations show an appreciable improvement of CISD dissociation energies.