A molecular mechanics calculation of the rotational potential experienced by methyl groups in molecular crystals is presented. Good agreement for seven out of the eight cases considered is obtained and the methyl group becomes a quantitative probe of its molecular environment. The calculation is used to gain insight into the coupling between methyl groups. For the smallest molecules, comparison between the generalized semiempirical calculation and a limited ab initio calculation on dimers of molecules suggests the simple rotational model employed is adequate, and that discrepancies between measured and calculated values are due to incorrect modeling of the atom-atom potentials.