Abstract
In this paper all approximations are discussed which lead to a quantum chemical model which is adequate for structural formulas known in classical organic chemistry. Neglect of the energy differences caused by changes of valence and dihedral angles, i.e. neglect of all nonbonding interactions, leads to the separation of the Hartree-Fock matrix in blocks of core electrons, nonbonding electron pairs, two center blocks of σ-bonds and delocalized π-electronic structures. Such a procedure can be formulated at all levels of sophistication - from the one electron approximation to the MC-SCF method. Even on the one electron level, the average error in heats of atomization (35.9 kJ/mol) is lower than that of the more complicated geometrical methods MINDO/3 (52.1 kJ/mol) and MNDO (39.6 kJ/mol). The procedure suggested is about two orders of magnitude more efficient than geometrical ones of the same level and can be, therefore, used for a study of reaction mechanisms of medium size systems (30-50 atoms) without large expense.
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