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Applications of the MBPT in the localized representation
Author(s) -
Kapuy Ede,
Bartha Ferenc,
Bogár Ferenc,
Csépes Zoltán,
Kozmutza Cornelia
Publication year - 1990
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.560380209
Subject(s) - diagonal , atomic orbital , perturbation (astronomy) , physics , perturbation theory (quantum mechanics) , ab initio , formalism (music) , diagrammatic reasoning , quantum mechanics , representation (politics) , statistical physics , electron , mathematics , geometry , art , musical , linguistics , philosophy , politics , political science , law , visual arts
Diagrammatic formulation of the MBPT is applied when the occupied and the virtual canonical orbitals are separately localized by unitary transformations. In this localized representation, due to the off‐diagonal Fock matrix elements, the perturbation operator contains extra terms generating the so‐called localization corrections. These corrections enter the perturbation energy in third and higher orders. Their magnitude depends on the type of localization, but they represent only a small fraction of the canonical corrections. The calculation of the localization corrections, however, does not need a significant amount of extra computer time. It is shown that by introducing an “order of neighborhood” local and nonlocal effects of the electron correlation can be separated and the contribution of the nonlocal effects can be neglected to a good approximation. Ab initio calculations have been carried out for the normal saturated hydrocarbons: C 2 n +1 H 4 n +4 and for the all‐trans conjugated polyenes C 2 n +2 H 2 n +4 . As to the ratio of the local and nonlocal corrections, it is shown that there is only a quantitative difference for these two kinds of systems (strongly or weakly localizable). Neglecting nonlocal effects, considerable amount of computer time can be saved.