Connection of an elementary class of parametric two-electron reduced-density-matrix methods to the coupled electron-pair approximations

摘要

Parametric two-electron reduced-density-matrix (p-2RDM) methods have recently been shown to exhibit accuracies greater than coupled cluster with single and double substitutions (CCSD) at a lower computational cost. In this paper we derive an elementary class of parametric 2-RDM methods with connections to the coupled electron pair approximations (CEPA). Three parametric 2-RDM methods p-2RDM are presented that correspond to the CEPA family where n = 1, 2, 3. We isolate the function distinguishing the stationary condition of the parametric 2-RDM methods from the nonlinear equations of CEPA. Calculations of energies, geometries, and harmonic frequencies show that p-2RDM and CEPA are very similar for a variety of closed-shell systems. Finally, each of the p-2RDM methods is extended to satisfy particle-hole symmetry by the removal of exclusion principle violating terms in the virtual space. These extensions denoted p-2RDM′ are shown to be essential for the proper dissociation of CH radical. Both p-2RDM and p-2RDM′ form an elementary class of parametric 2-RDM methods with accuracy like CCSD; more general parametric 2-RDM methods [D.A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)] have an accuracy approaching CCSD with full triple excitations.View full textDownload full textKeywordselectron correlation, reduced density matrices, N-representability conditions, particle-hole duality, variational principleRelated var addthis_config = { ui_cobrand: "Taylor & Francis Online", services_compact: "citeulike,netvibes,twitter,technorati,delicious,linkedin,facebook,stumbleupon,digg,google,more", pubid: "ra-4dff56cd6bb1830b" }; Add to shortlist Link Permalink http://dx.doi.org/10.1080/00268976.2012.695027