Electronic Correlation in Atoms and Molecules

The Ornstein–Zernike equation in molecular electronic structure theory

“…This leads to an over-estimation of correlation effects. The exact independent electron pair theory, IEPA [12,13], overestimates the correlation energy by around 15% (see below), implying that the lack of treatment of inter-pair correlations raises the MP2 correlation energy by a comparable amount.…”

“…This results in a spurious elevation of the virtuals, an increase in the energy denominators, and, hence, an under-estimation of correlation effects [15]. The spurious interactions present in the Møller-Plesset denominator are explicitly subtracted in Epstein-Nesbet (EN) perturbation theory [13,[16][17][18]. EN2 performs very well for systems of two (correlated) electrons such as H 2 or LiH [15].…”

Electronic pair correlation in atoms and molecules

“…In the extensive literature on Brueckner's theory following these papers, this equation is referred to as a Bethe-Goldstone equation. Surveys of this literature have been published by Thouless [29] In my review article [24], it was shown that the Brueckner theory could be expressed in terms of localized Hartree-Fock orbitals as well as of the canonical Hartree-Fock orbitals usually considered. The latter in the case of a molecule or solid extend over the entire system, while localized orbitals can be associated with electron pair bonds, lone pairs, and inner shells as considered by LennardJones [3].…”

Perturbed atoms in molecules and solids: The PATMOS model

“…[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] Such an approach was introduced as early as in 1964 by Sinanoǧlu 31 and Nesbet. 32 However, the real breakthrough for local correlation models can be attributed to an idea proposed by Pulay,33 and later implemented by Saebø and Pulay [34][35][36][37] for Møller-Plessett perturbation theory. Their approach implies that excitations from localized molecular orbitals are restricted to subspaces of projected atomic orbitals which are spatially close to the localized orbitals.…”