An evaluation of the MM+ force field

Hyperchem's MM+ force field, based on Allinger's MM2 is described and evaluated with respect to other MM2 variants, in terms of rotation barriers, conformational energy differences, and conjugation. Its ability to take missing parameters into account is also evaluated with respect to the Dreiding force field. This evaluation also intends to clearly separate the two different force fields MM+(91) and MM+(**) hiding under the MM+ denomination. It is shown that, whereas MM+ proves to be generally robust, caution must be the rule when dealing with conjugated molecules, particularly with heteroaromatics.