STATIONARY COUPLED-CLUSTER APPROACHES TO MOLECULAR-PROPERTIES - A COMPARATIVE-STUDY

We show in this paper that the use of an extended coupled-cluster func tional leads to a more desirable stationary-response formulation for m olecular properties than that obtained by the use of an expectation-va lue functional. We make a detailed formal and numerical analysis of re lative suitability of both the functionals. In particular, we investig ate the extent of size-separation error using a truncated expectation- value functional and examine the relative accuracy and convergence of the two functionals in extracting molecular properties. We have also c ompared the results obtained by the use of these two functionals with the nonvariational coupled-cluster results.