Evidence for phase space transitions in excited triatomic molecules

At high levels of vibrational excitation it is not realistic to examine the energy levels of molecules state by state. A more coarse grained measure is required. The use of the free energy is illustrated by application to a realistic model Hamiltonian for the stretch vibrations of triatomic molecules, both exact and variational (self-consistent) computations are reported. A transition from a localized to an equipartitioned distribution of energy (as a function of mean excitation energy) is noted and discussed.