On the dilemma of the potential energy calculations with geometry optimization for spectroscopic purposes: application to a two-dimensional problem

Theoretical techniques for determining the potential energy functions for t he double methyl rotation with full optimization of geometry an considered. It is emphasized that, although the methyl groups apparently lose their C- 3 symmetry because of environmental effects, the dynamical three-fold symme try properties must be conserved, The problem is examined in the case of bi acetyl as an example of a two-C-3 rotor problem and applied to the ab initi o determination of the lowest levels of the torsional far-infrared spectrum of this molecule. After recording the dynamically symmetric approach, whic h makes the variational and symmetry principles compatible, it is shown tha t the methods which average the energy of the energetically 'equivalent' co nformations furnish the closest solutions to those of the correct approach. (C) 1999 Elsevier Science B.V. All rights reserved.