Semiempirical MO methods: the middle ground in molecular modeling

Abstract

Semiempirical methods occupy an important middle ground between molecular mechanics and ab initio MO calculations in the repertoire of methods available for studying the structures, properties and reactions of molecules. They have a unique combination of speed and generality which makes it possible to study many chemical systems which are beyond the reach of classical force fields and too large for ab initio MO methods. Indeed, semiempirical calculations are often the first computational technique to be applied to a chemical problem. Three examples where semiempirical MO calculations have provided significant mechanistic insight are the cylcopropylcarbinyl cation, porphyrin structure and dynamics, and the role of CH hydrogen bonds in polymer miscibility. In each case Semiempirical calculations have been at the fore, and their results have been generally confirmed by subsequent ab initio calculations and experiment.