Defective modelling of chaotic motions on empirical potential energy surfaces

Abstract

The progression from regular to chaotic behaviour with increasing vibrational energy is examined for two unimolecular reactions, CH₃NC ⇌ CH₃CN and NCNC ⇌ NCCN. The potential energy surfaces used are, respectively, a piecewise empirical construction and an ab initio numerical surface. It is found that in the former case, the motions never become chaotic in the appropriate energy range, but in the latter, they seem to be approaching that ideal condition. The reasons for this difference are subject to speculation at the present time, but there seems to be a strong impediment to randomisation of energy in one case that is not present in the other. An attempt is made to formulate a semi-quantitative measure of chaotic behaviour in these reactions. Until this problem with synthetic potential energy surfaces can be resolved, these results have important consequences for the numerical modelling of larger polyatomic systems, up to and including such problems as protein folding.