Second-order correction to perfect pairing: An inexpensive electronic structure method for the treatment of strong electron-electron correlations

We have formulated a second-order perturbative correction for perfect-pairing wave functions [PP(2)] based on similarity-transformed perturbation techniques in coupled cluster theory. The perfect-pairing approximation is used to obtain a simple reference wave function which can qualitatively describe bond breaking, diradicals, and other highly correlated systems, and the perturbative correction accounts for the dynamical correlation. An efficient implementation of this correction using the resolution of the identity approximation enables PP(2) to be computed at a cost only a few times larger than that of canonical MP2 for systems with hundreds of active electrons and tens of heavy atoms. PP(2) significantly improves on MP2 predictions in various systems with a challenging electronic structure. (c) 2006 American Institute of Physics.