On the importance of higher order anharmonic molecular couplings

The dynamics of molecular vibrational dephasing (IVR) are governed by the coupling structure of the vibrational Hamiltonian. We use a recently developed factorization model for normal coordinate potentials to compare the importance of direct high order couplings to both direct low order couplings and low-order coupling chains. The importance of high order couplings is characterized as a function of the vibrational energy and of molecular size. Particular attention is paid to phase cancellation effects in coupling chains, and to correlations between vibrational modes depending on their localization length. To account properly for IVR at high densities of states and long times, high order couplings must be taken into account.