Zero-point energy constraints in RRKM and non-RRKM molecules

The TRAPZ method for zero-point energy (ZPE) preservation (Lim and McCormack, J. Chem. Phys., 1995, 102, 1705) is generalised to molecular systems of non-zero angular momentum. The method is shown to conserve linear momentum, angular momentum and total energy. It is also found to preserve the intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behaviour of a dissociating Al3 cluster. The TRAPZ rate coefficients are lower than those calculated for classical ensembles, since regions of phase space with less than ZPE can no longer react. As required, the RRKM rate coefficients are upper bounds to the TRAPZ rate coefficients. The TRAPZ reaction threshold is higher than the classical (asymptotic-limit) product ZPE due to fluctuations in instantaneous normal mode ZPEs. Rotationally hot Al2 is produced as Al3 ZPE bending motion is converted to Al2 angular momentum. The TRAPZ method does not preserve the non-RRKM behaviour of HNC isomerisation.