PREDICTION OF GEOMETRIES AND INTERACTION ENERGIES OF COMPLEXES FORMEDBY SMALL MOLECULES USING SEMIEMPIRICAL AND AB-INITIO METHODS

The accuracy of the semiempirical quantum mechanics methods (AMI and P M3), and the ab initio methods (6-31G* and MP2/6-31G**) in predicting intermolecular geometries and interaction energies have been evaluate d by detailed studies of 17 bimolecular complexes formed by small mole cules. Comparisons between calculated and experimental geometries for 12 complexes are presented. It was found that AM1 gave reasonably good predictions of the geometries of complexes such as CH4 ... CH4, which have very weak interactions, but it is not as good as other methods i n predicting intermolecular geometry for complexes where hydrogen bond ing interactions play an important role. This is consistent with its i nability to reproduce the charge transfer in the formation of hydrogen bonds in these complexes. PM3 is able to predict intermolecular geome tries for most complexes, including those with hydrogen bonding; its. major flaw is its tendency to overestimate the strength of the interac tions between hydrogen atoms. Care should be taken therefore in using PM3 to study complicated molecular systems with multiple hydrogen atom interactions and the method's weakness in handling complexes in which electrostatic forces are important should also be noted. Among ab ini tio methods, both the 6-31G* and the MP2/6-31G** were found to outper form AM1 and PM3 in prediction of intermolecular geometry. Both of the se ab initio methods showed excellent consistency in geometry predicti on for most of the complexes studied, although MP2/6-31G* is better t han 6-31G*. It is noted that the MP2/6-31G** did not produce the corr ect geometry for the CO2 ... HF complex. For 12 complexes for which ex perimental geometry data are available, AMI, PM3, 6-31G*, and MP2/6-3 1G* successfully predicted the geometry in 10, 12, 12, and 11 cases, respectively. The average errors given by AM1 in the predicted intermo lecular distances were 0.264, 0.272, 0.091, and 0.061 angstrom, respec tively. In comparison to the ab initio methods, AM1 and PM3 commonly u nderestimated the molecular interaction energy in such complexes by ap proximately 1-2 kcal mol-1.