Bound and continuum vibrational states of the bifluoride anion

The energies of the bound vibrational states and energy density spectra of the continuum vibrational states of FHF− are calculated, ‘exactly’ and ‘adiabatically’, using a new ab initio (CCSD(T)) potential energy surface. Statistical properties of the bound states are probed in terms of the density of states and nearest neighbor level spacing distributions (NNSD). Importantly, the approximate ‘adiabatic’ densities coincide nearly quantitatively with their ‘exact’ counterparts. A quantitative fitting of the NNSDs is achieved with a new empirical modification of the Wigner distribution.