Approximate account of connected triply and quadruply excited clusters in single reference coupled cluster theory via cluster analysis of projected unrestricted coupled cluster with single and double wave function

The projected unrestricted coupled cluster wave function with singly and doubly excited clusters (PUCCSD) has been used as a source of information about the T3 and T4 cluster contributions in the externally corrected CCSD approach. In order to remove the spin contamination of the UCCSD wave function the group invariant average of the unitary group was adapted. Preliminary results presented here for the DZ H8 model system show surprisingly good agreement with the exact results, particularly in the case of the energy contribution of the T4 cluster to the quasidegenerate region with errors of less than 0.1 milihartree.