Hydrogen bonding in MM2
Hydrogen bonding is qualitatively accounted for in the early versions of the MM2 program, but not quantitatively. Experimentally, the hydrogen bonds are somewhat shorter and stronger than calculated by MM2. This has been corrected now in MM2(87), by reducing the van der Waals radius of the hydrogen involved in hydrogen bonding for that interaction only, and by increasing the van der Waals' attraction between the hydrogen and the various electronegative atoms to which it can hydrogen bond by about 1–3 kcal/mol, depending on the particular atoms involved. It is now possible to reproduce reasonably well ab initio calculations on simple amides and the methanol dimer, and experimental data on compounds in which a hydroxyl hydrogen is hydrogen bonded to an alcohol, an alkyl chloride, or to a carbon–carbon double bond.