A semiclassical approach to the torsional tunneling splitting patterns in G6(EM), G12, G18, and G36(EM) molecules

A semiclassical method to predict the torsional splitting patterns in internally rotating molecules of G6(EM), G12, G18, and G36(EM) symmetries is described. To this purpose use is made of rotating axis systems denoted “Corioless”, in which the contribution to the torsional angular momentum from specific vibrational motions of the molecule, or part of it, vanishes. It is confirmed that the actual torsional splitting patterns in degenerate or perpendicular vibrational states can be predicted analytically and are determined by torsional Coriolis coupling. Rules stating the conditions required to generate limit splitting patterns (regular and inverted) are formulated in a quite general way and derived for the first time in the case of G18 molecules.

An ab initio potential energy surface for the formic acid dimer: zero-point energy, selected anharmonic fundamental energies, and ground-state tunneling splitting calculated in relaxed 1–4-mode subspaces

We report a full-dimensional, permutationally invariant potential energy surface (PES) for the cyclic formic acid dimer.