Improvement of the standard method for solving the problem of internal rotation in molecules with symmetrical tops

Drawbacks of the standard method for solving the problem of internal rotation (IR) in molecules with symmetrical tops were examined. The drawbacks can be eliminated by taking into account the higher harmonics of the IR potentials. A new calculation procedure based on the approximation of the lower portion of the potential curve by the equation V(phi) = 1/2 V-n*(1 - cos n phi), which describes adequately the arrangement of the torsion levels, was proposed. The successful implementation of the proposed procedure showed that the procedures used currently are incorrect as well as the Herschbach procedure, which includes a systematic error at each iteration step.