Abstract
It is shown that all valence semiempirical theories are incapable of treating screening effects in a consistent fashion. This is due to their development in terms of the zero differential overlap assumption which leads to an imbalance of interactions in off‐diagonal matrix elements. A new formulation of semiempirical theories is presented which is based on an expansion of the Fock operator in terms of averaged potentials centered either on atoms or in bonds. STO‐3G calculations are carreid out to demonstrate that our averaging procedure is a reasonable one. The form of current semiempirical theories is easily recovered although many of the integrals are ascribed a somewhat different meaning. Screening now arises naturally as does a valence index and obvious corrections that should be made upon changes in hybridization. The resonance integral is given a somewhat modified interpretation and on this basis, a unified treatment of both ground and excited state properties is developed. A new method of calculati...
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