Comparison of Single Point Ab initio Energies Calculated Using 3-21G and AM1 Geometries
TEXAS UNIV AT AUSTIN DEPT OF CHEMISTRY
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Single pint ab initio calculations for number of molecules, using various procedures with 3-21G or AM1 geometries, show that the AM1-based results seem to be better for high level ab initio procedures and for studies of reactions, as well as requiring less computing time. Current ab initio methods usually give good estimates of heats of reaction if a large basis set is used and if allowance is made for electron correlation, e.g. by Moeller-Plesset MP perturbation theory. However, calculations of this kind require too much computing time to be carried out rigorously, i.e. with full geometry optimization, for any but small molecules. It has therefore become a standard practice in such cases to calculate the geometries by a simple and correspondingly cheap method, usually the 3-21G model, and then to carry out single point calculations by the high level procedure. The same procedure is also commonly used in studies of the Transition States TS of reactions where the cost of calculations by a correlated high level procedure is even more forbidding. Reprints.