CALCULATION OF FORMAL STERIC ENTHALPY WITH MM2

Steric energies calculated by molecular mechanics are used to estimate enthalpies of formation and differences and double differences of enthalpies of formation. The underlying principles and the precautions necessary to obtain valid results are analyzed. To compare results of calculations with two different force fields SE values are of little use, but they may be normalized to formal steric anthalpy (FSE) values which do provide a direct and unbiased comparison. Procedures are described for calculating FSE values with the MM2 and MMS force fields. It is strongly recommended that results of molecular mechanics calculations be reported in terms of FSE values so that calculations in different laboratories with the same or with different force fields may be directly compared.