On the dilemma of potential energy calculations with geometry optimization for spectroscopic purposes

Theoretical techniques for determining the potential energy functions for methyl rotation with full geometry optimization are examined. It is shown that, although the methyl group loses C3 symmetry because of the environmental effects reflected in the optimization, the threefold permutational property of three identical particles imposes C3 symmetry on the potential energy function. Partial solutions to this dilemma are given for the cases of one- and two-C3 rotor problems.