The Effect of Intramolecular Interactions on the Transferability Properties of Localized Descriptions of Chemical Groups

Starting from a localized description of molecular wavefunctions, approximate descriptions of chemical groups are obtained. The degree of conservation of these representations is verified over a fairly large number of compounds by using different tests, from which it is seen that the influence of intramolecular interactions is strong enough to necessitate the inclusion of these effects in the description of molecular systems in terms of transferable models of groups. Further analyses demonstrate that intragroup interactions can be approximated, to a good degree of precision, by classical interactions only. The usefulness of a zeroth order representation of a group in terms of nonorthogonal transferable models of localized orbitals, coupled to an evaluation of classical intragroup interactions, is discussed, and a simple computational method for the evaluation of the changes in reaction barriers due to substitution of chemical groups in a given molecular framework is briefly outlined.