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An ab initio form of classical valence‐bond theory
Author(s) -
McWeeny Roy
Publication year - 1999
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/(sici)1097-461x(1999)74:2<87::aid-qua5>3.0.co;2-e
Subject(s) - valence bond theory , atomic orbital , chemistry , core electron , molecular orbital theory , generalized valence bond , modern valence bond theory , valence (chemistry) , ionic bonding , valence electron , ab initio , valency , molecular orbital , orbital hybridisation , computational chemistry , molecule , atomic physics , electron , ion , quantum mechanics , physics , linguistics , philosophy , organic chemistry
In this work the classical valence‐bond (VB) theory of the 1930s is recast in a fully ab initio modern form. The basic premises are simply (i) that valency is associated with singly occupied orbitals on the constituent atoms of a molecule, more tightly bound electrons being assigned to a “core,” (ii) that the valence electrons may be well described using resonance mixtures of alternative spin‐paired structures; and (iii) that the structures used are not confined to a single orbital configuration but may include those of “ionic” type (in which some orbitals are doubly occupied and others are empty) and indeed spin‐coupled functions based on arbitrary selections of nonorthogonal orbitals. The resultant multiconfiguration VB theory can thus accommodate all the basic pictorial concepts introduced by Pauling, Wheland, and others in the early days of quantum chemistry. The approach is mathematically simple, here being derived from first principles, and permits not only the optimization of structure coefficients but also that of the forms of both core and valence orbitals. An illustrative example, in which the benzene and pyridine molecules are discussed, along with their positive and negative ions, confirms the generality and feasibility of the approach. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 87–96, 1999