Ground and asymmetric CO-stretch excited state tunneling splittings in the formic acid dimer.

There has been some controversy concerning the assignment of measured tunneling splittings for the formic acid dimer in the vibrational ground state and the asymmetric CO-stretching excited state. The discussion is intimately related to the question whether the fundamental excitation of the CO-vibration promotes or hinders tunneling. Here we will address this issue on the basis of a five-dimensional reaction space Hamiltonian which includes three large amplitude coordinates as well as two harmonic modes whose linear superposition reproduces the asymmetric CO-vibrational mode. Within density functional theory using the B3LYP functional together with a 6-311++G(3df,3pd) basis set we obtain a ground state tunneling splitting which is about 2.4 larger than the one for the CO-stretching excited state.