Economical description of electron correlation

TitleEconomical description of electron correlation
Publication TypeBook Chapter
Year of Publication2007
AuthorsBytautas L, Ruedenberg K
EditorWilson AK, Peterson KA
Book TitleElectron Correlation Methodology
CityWashington, DC
PublisherAmerican Chemical Society

Methods are presented for reducing the computational effort required to account for electron correlation in large mols. The correlation recovery is divided into two stages: Recovery of zeroth-order dynamic correlations and dynamic correlation refinements. The former is achieved through MCSCF wavefunctions related to full valence spaces. A method is developed for the a-priori elimination of the configurational deadwood from such wavefunctions. Although orbital-independent, the procedure is most effective in conjunction with the use of appropriately localized MOs. Applications to the mols. HNO, OCO and NCCN show that drastic shortenings of the CI expansions (e.g., from 776,316 to 43,038 SDTQ determinants in NCCN) raise the energy by less than 1 mh. For the detn. of the dynamic correlation energy, on the other hand, a model is developed that leads to an additive expression in terms of pair populations of localized orbitals multiplied by correlation energy contributions of electron pairs within and between such orbitals. This simple formula predicts the valence correlation energies of about 50 org. mols. with a mean abs. deviation of about 2 kcal/mol. Contributions of nonadjacent localized MOs are found to be near negligible. [on SciFinder (R)]