Effect of the straight path approximation and exchange forces on vibrational energy transfer

A wide variety of theories exist for calculating fundamental molecular collision processes such as line broadening or energy transfer rates. All of these theories involve evaluation of a ``phase'' integral or Fourier transform of the intermolecular potential. In this paper we consider two major sources of error that can occur in the evaluation of phase integrals. ``Universal'' phase integrals calculated in a straight path model are compared with calculations based on trajectories determined by solution of classical orbit equations. Serious discrepancies are pointed out and practical computational alternative is suggested. A simple empirical model for exchange forces is introduced in calculations of linewidths and V–V transfer for hydrogen halides in a CO2 atmosphere. These calculations show that exchange forces can have a large effect but that the simple procedure of adding ``a short range'' contribution to a cross section calculated from attractive multipolar forces gives a totally incorrect picture.