AN ABINITIO CALCULATION OF THE NONADIABATIC EFFECT ON THE TUNNELING SPLITTING IN VIBRATIONALLY EXCITED (HF)2

We have previously determined an analytical ab initio 6-dimensional potential energy surface for the HF dimer, and used it in the calculation of rotation-vibration energies. In the first excited states of the HF stretching vibrations, the tunneling splitting is found experimentally to be much less than in the vibrational ground state, and it has been suggested that a vibrational nonadiabatic coupling, caused by the tunneling kinetic energy, could significantly contribute to the lowering of the tunneling splitting. We calculate this effect, using vibrational wavefunctions derived from our ab initio surface, and find it to be negligible. The adiabatic (diagonal) effect of the tunneling kinetic energy is important, but it is not large enough to completely account for the reduction in the tunneling splitting. © 1991.