Perturbative Approximations to Spin Flip Coupled Cluster Singles Doubles - Application to Bond Breaking Problem

Spin flip equation of motion coupled cluster (EOM-SF-CC) can correctly treat situations involving electronic degeneracies or near degeneracies, e.g, bond breaking, di- and tri-radicals etc. However, for large systems EOM-SF-CC (even in single and double excitations) is computationally prohibitively expensive. Therefore, earlier approximations to EOM-SF-CC methods such as spin flip configuration interaction singles with perturbative doubles (SF-CIS(D)) has been proposed. In this talk, I will present a new perturbative approximation to EOM-SF-CC, which has been found to be more accurate than the earlier SF-CIS(D). The capabilities, advantages and timings of the new approach will be demonstrated through bond breaking problems, oxirane ring opening and cyclo-butadiene automerization reaction.