Analytic Route to Tunneling Splittings Using Semiclassical Perturbation Theory

We
present an efficient, analytical, and simple route to approximating
tunneling splittings in multidimensional chemical systems, directly
from ab initio computations. The method is based
on the Wentzel–Kramers–Brillouin (WKB) approximation
combined with the vibrational perturbation theory. Anharmonicity and
corner-cutting effects are implicitly accounted for without requiring
a full potential energy surface. We test this method on the following
three systems: a model one-dimensional double-well potential, the
isomerization of malonaldehyde, and the isomerization of tropolone.
The method is shown to be efficient and reliable.