SEMIEMPIRICAL MO METHODS - THE MIDDLE GROUND IN MOLECULAR MODELING

Semiempirical methods occupy an important middle ground between molecu lar mechanics and ab initio MO calculations in the repertoire of metho ds available for studying the structures, properties and reactions of molecules. They have a unique combination of speed and generality whic h makes it possible to study many chemical systems which are beyond th e reach of classical force fields and too large for ab initio MO metho ds. Indeed, semiempirical calculations are often the first computation al technique to be applied to a chemical problem. Three examples where semiempirical MO calculations have provided significant mechanistic i nsight are the cylcopropylcarbinyl cation, porphyrin structure and dyn amics, and the role of C-H hydrogen bonds in polymer miscibility. In e ach case semiempirical calculations have been at the fore, and their r esults have been generally confirmed by subsequent ab initio calculati ons and experiment. (C) 1997 Elsevier Science B.V.