Use of a size-consistent energy functional in many electron theory for closed shells

If the ground state wave-function Ψgr is written as Ψgr = Φo+X, with X as the correlation part satisfying (Φo|x) = 0, and x expressed as an expansion in terms of pair, pair-pair etc. cluster functions, then the expectation value of the energy E = (Ψgr|H|Ψgr)/(Ψgr|Ψgr) has the property that the normalization term in the denominator completely cancels the unlinked part of the numerator, as noted by Sinanoglu. We use Cizek's coupled-pair ansatz Ψgr = exp(T2) Φ0 for transcribing Sinanoglu's expansion in a many-body language to study the behaviour of the size-consistent (linked) energy functional thus generated. For calculating the matrix-elements of the cluster components of T, we use two recipes: (1) a variational determination of the cluster components using Euler's principle for the energy functional akin in spirit to the Varied Portion Approach (VPA) of Sinanoglu and (ii) a nonvariational determination of the cluster components using the conventional coupled-cluster theory. Results are presented for model test systems and are compared with variational CI and nonvariational coupled-cluster values. It has been observed that the values obtained from the size-consistent energy functional from the cluster components obtained from methods (i) and (ii) are quite close and both compare well with the nonvariational coupled-cluster results. Some useful simplifications afforded by the VPA are also indicated. A brief perspective of the method vis-a-vis other related theories is also given