A test of RRKM theory against numerical simulation for classical chain molecules. I. Method and preliminary results

Abstract

A systematic investigation of the internal part of the unimolecular reaction dynamics of one-dimensional chain molecules has been undertaken to examine the validity of RRKM theory. The basic aims, means and scope of this program of research are reported here together with results for uniform chains. Classical mechanics is used and the atoms in the chain interact by Morse pair potentials acting between neighbouring particles. The simulation method described here consists of a very efficient Monte Carlo sampling of microcanonical initial states coupled with a predictor-corrector molecular dynamics algorithm. Time-dependent and best fitting time-independent decay rate coefficients can be extracted from the set of lifetimes to dissociation observed in the simulation. These are then compared to the corresponding rate coefficients predicted by the usual form of RRKM theory where anharmonic contributions ot the density of states are neglected or to a more rigorous implementation where the anharmonic contributions are accounted for. Calculations for uniform chains reported here establish a pattern of deviations from RRKM prediction which will be explored and explained in greater detail in this program of research.