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Fifth‐order constant denominator perturbation theory within a localized bond model: Contributions from triple and quadruple excitations
Author(s) -
Cullen J. M.
Publication year - 1989
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560350407
Subject(s) - perturbation (astronomy) , delocalized electron , perturbation theory (quantum mechanics) , atomic orbital , wave function , chemistry , physics , quantum mechanics , computational chemistry , atomic physics , molecular physics , electron
Abstract The contributions of the triple and quadruple excitations to the fifth‐order perturbation energy for the perturbation configuration interaction using localized orbitals ( PCILO ) method are derived. This completes the development of a fifth‐order constant denominator perturbation theory initiated in a previous paper [5] with the single and double excitations. This theory is tested on molecules containing strained ring geometries, stretched bonds, strongly polarized bonds, and delocalized pi systems: cases where the starting zero order reference wave function poorly describes the system. Although the perturbation expansions turn out to be slowly convergent, the Padé approximant taken from an energy series which itself is constructed from Padé approximants provides results accurate to within a few kilocalories/mole of benchmark calculations. Computational times as in the original PCILO procedure remain proportional to N 3 , where N is the number of bonds.