PARTITIONED EQUATION-OF-MOTION COUPLED-CLUSTER APPROACH TO INDIRECT NUCLEAR SPIN-SPIN COUPLING-CONSTANTS

Two approximations to equation-of-motion coupled cluster theory for se cond-order properties are introduced that increase the computational e fficiency of the approach without forsaking the accuracy of the method . Using partitioning techniques the doubles-doubles block in the EOM H amiltonian is replaced by a diagonal approximation, and in addition th e CCSD ground state may be replaced by its analog from first-order per turbation theory. It is shown that partitioning hardly affects the acc uracy of the approach. The Fermi contact contribution to the coupling constant is found to be most sensitive to the inclusion of ground stat e correlation.