Explicit Electron Correlation by a Combined Use of

Abstract. In MP2-GG « theory, the second-order Meller-Plesset (MP2) energy is recovered by a combined Gaussian-orbital—Gaussian-geminal (GG) expansion of the first-order wave function. While the restriction of geminals to doubly occupied orbital pairs (MP2-GG0) provides a modest improvement on standard MP2 theory, their inclusion also in all singly-excited (MP2-GG1) and doubly-excited (MP2-GG2) pairs recovers essentially all of the correlation energy in small (double-and triple-zeta) basis sets. For several small systems, our MP2-GG « energies represent the best MP2 energies reported in the literature