Survey of Selected SMx Results

In fact, very large basis set, correlated calculations suggest that the CH tautomer is favored over the NH in the gas phase by only 6.2 kcal/mol. Assuming an approximately equal mixture of the two tautomers in aqueous solution,3i3 the AMI-SMIa model is within about 1 kcal/mol of correctly predicting the relative free energies of solvation. The more general AM1-SM2 model is not quite as accurate in its consideration of heteroatom-bound protons, and it thus provides a somewhat poorer prediction in this instance, although it remains closer than any of the other models. 

The last example illustrates the difficulties involved in studying the effects of solvation on conformational equilibria and chemical reactions. Whereas it is important to have a solvation model that performs reasonably accurately (e.g., the 0.6 kcal/mol mean error for neutral solutes observed with AM1-SM2) such accuracy is of little use when the gas phase surface of the same system contains significantly larger errors. 

This section provides further information on the SMx parameterizations and discusses examples of special results to illustrate the kinds of problem that can be considered. 

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