Pair-Only Coupled Cluster from a Viewpoint of Geminal Wave Functions:
Accurate Bond Dissociation Curves at MP2-like Computational Cost
Dr. Peter A. Limacher
Department of Chemistry and Chemical Biology, McMaster University, Hamilton (ON)
Antisymmetrized product of electron pairs (geminals) have been recognized as conceptually interesting and highly accurate wavefunctions since the early days of quantum chemistry. In contrast to the conventional orbital approach, geminals make explicit use of the two-particle nature of the Hamiltonian. Although very appealing from a fundamental point of view, until recently there were no computationally tractable approaches that did not impose severe restrictions on the geminals (e.g. strong orthogonality). Applying a different, more mildly set of approximations, conceptually related to coupled cluster with solely closed-shell pair excitations, it was possible to surmount this bound and use non-orthogonal geminals without sacrificing the mean-field like computational cost.
We could show that this method is highly reliable for bond dissociations and accurately models all effects of strong electron correlation, yet fails to fully describe weak (dynamical) correlation. To that end, an easy and computationally inexpensive perturbation extension to the method is applied yielding results close to full CI calculations, but at the same computational scaling as ordinary MP2.
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 P.A. Limacher, P.W. Ayers, P.A. Johnson, S. De Baerdemacker, D. Van Neck, P. Bultinck, J. Chem. Theory Comput. (in press).