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Correlation effects in the low–lying excited states of the PPP models of alternant hydrocarbons. I. Qualitative rules for the effect of limited configuration interaction
Author(s) -
Čížek J.,
Paldus J.,
Hubač I.
Publication year - 1974
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.560080610
Subject(s) - configuration interaction , excited state , hamiltonian (control theory) , electronic correlation , singlet state , chemistry , valence (chemistry) , lying , electron configuration , quantum mechanics , atomic physics , computational chemistry , physics , electron , mathematics , mathematical optimization , medicine , radiology
Simple rules for an estimate of the correlation effects in the low‐lying states of alternant hydrocarbons, as described by the Pariser–Parr–Pople Hamiltonian, are formulated. These rules are based on the alternancy and spin symmetry classification of states in both strongly and weakly correlated limits and on the valence bond characteristics of those states in the fully correlated limit. It is shown that the largest effect of the electron correlation will be found for the singlet “minus” states (using Pariser's classification of the alternancy symmetry species), a smaller effect for the triplet “plus” states, and a much smaller effect for the remaining states. These rules are exemplified by limited CI calculations including all monoexcited and all mono‐ and bi‐excited configurations, respectively, for a number of π‐electronic systems. In view of these rules the success of the PPP model in the monoexcited CI approximation may be understood.