Coupled pair approximation

Williams HL, Szalewicz K, Moszynski R, Jeziorski B (1995) Dispersion energy in die coupled pair approximation with noniterative inclusion of single and triple excitations. J Chem Phys... 

In the coupled pair approximation,160 all diagrams which can be formed by considering products of disconnected double-excitations are summed to infinite order. Thus in fourth-order, the coupled pair approximation includes the linked double-excitation and linked quadruple-excitation diagrams shown in Figure 6 and Figure 8, respectively. The coupled pair approximation may be represented diagrammatically as follows ... 

Coupled Cluster (cc) expansions Approximations based on CC expansions [3, 91,92] include ccd, ccsd, ccsdt, etc., and their multireference variants mr-CCD, MR-ccsD, MR-ccsDT, etc. The CCSD approximation is also known as the coupled pair approximation (cpa) [59] or the coupled pair many electron theory (CPMET) [20,21,93]. 

Sakai et al. used several advanced post-Hartree-Fock methods (complete active space SCF, multi-reference singly- and doubly-excited conguration interaction, and multi-reference coupled pair approximation) in studies of low-lying electronic states of TiCl and ZrCl [209]. They found that the ground state for TiCl was and the lowest excited states were and (in that order). For ZrCl it was the doublet that was the ground state, with the excited states and S . The computed spectroscopic constants were reasonably close to the experimental ones. 

Ahlrichs, R. and Scharf, P., The coupled pair approximation, Adv, Chem. Phys. 67 501 (1987). Emphasis is on the ideas behind CEPA-like methods and on their application. (CEPA discussed in this book corresponds to CEPA>2 in this article). 

---