Quasiclassical trajectory calculations compared to quantum mechanical reaction probabilities, rate constants, and activation energies for two different potential surfaces for the collinear reaction H2+I→H+HI, including dependence on initial vibrational state

Abstract

Quasiclassical trajectory studies of the H+H2 reaction on an accurate potentialenergy surface. III. Comparison of rate constants and cross sections with experiment Quasiclassical trajectory studies of the H+H2 reaction on an accurate potential energy surface. II. Effect of initial vibration and rotation on reactivity Quantum mechanical calculations are compared to quasiclassical trajectory forward (QCT) calculations for the collinear. endoergic reaction Hlnl)+I->H+HI for two different potential energy surfaces, a rotated-Morse-curve (RMC) surface and the semiempirical valence-bond surface of Raff et al. Vibrationally stateselected reaction probabilities and rate constants and Arrhenius parameters are presented. Thermally averaged rate constants and their Arrhenius parameters are also given. For one of the potential energy surfaces, quasiclassical trajectory reverse histogram (QCTRH) calculations were also performed. The results show that classical mechanics and quantum mechanics are in significant qualitative agreement for state-selected properties. Specifically, for the n I = 0 state of the Raff et al. surface the quantum mechanical reaction probabilities are very small (less than 0.005) and the QCT method predicts this state to be totally non-reactive. For all other states on both surfaces quantum mechanics and QCT and QCTRH results all agree that reaction probabilities attain much higher values (up to 0.85). For both surfaces quantum mechanical and QCT results predict that excited vibrational states make significant contributions to the thermal reaction rates, although the methods disagree as to which vibrational state is quantitatively most important. Quantitative agreement with quantum mechanical results is obtained only with the QCTRH method for thermally averaged rate constants (agreement within 2%) and with both QCT and QCTRH methods for the Arrhenius parameters (agreement within a few tenths kcal mol-1 for activation energy). However, to achieve such agreement the QCT method had to be suitably modified to correct unphysical discrepancies in the threshold energy region. We present tables of these and many other results as a function of temperature. These should be useful in assessing the validity of trajectory studies of various kinds of reaction attributes under conditions where they are used to interpret experiments.

Zero-point energy, tunnelling, and vibrational adiabaticity in the Mu + H2reaction

“…It has been known for a long time that QCT calculations are not quantitatively reliable for calculating thermal rate constants because of their inadequate treatment of threshold behaviour [28][29][30]. It has also been known for a long time [31,32] that imposing a ZPE constraint in the product region can improve the agreement between QCT and QM results. In a very relevant study of the Mu + H 2 reaction [33], it is stated in the abstract that 'We find the backward trajectory calculations are more accurate, as would be expected since the dynamical bottleneck occurs toward the products in the Mu + H 2 reaction.…”

Coupled-surface investigation of the photodissociation of NH3(Ã): Effect of exciting the symmetric and antisymmetric stretching modes

“…Algorithmically though, maintaining ZPE in classical trajectory simulations has been an unmet challenge for decades. 17,35,[43][44][45][46][47][48][49][50][51][52][53] Due to severe shortcomings of earlier ZPE maintenance methods, 44,45,48 a new scheme, called trajectory projection onto ZPE orbit ͑TRAPZ͒, was proposed by Lim and McCormack 47 and McCormack et al 49,51 It was assumed to remove spurious features from the dynamics, but it was recently found 35 that it leads to unphysical results when used to model the photodissociation dynamics of NH 3 ͑Ã ͒. Furthermore, we obtained unphysical results when we did not maintain ZPE.…”

Zero-Point Energy Constraint for Unimolecular Dissociation Reactions. Giving Trajectories Multiple Chances To Dissociate Correctly

“…Models have been proposed and used for addressing the unphysical flow of ZPE in classical chemical dynamics simulations. − These unphysical ZPE dynamics can artificially enhance the rate of intramolecular vibrational energy redistribution (IVR) and also lead to the formation of reaction products without ZPE . For bimolecular reactions, trajectories which do not have ZPE in the reaction products are often discarded. − This may be either a soft or hard ZPE constraint, , where the former discards trajectories if the sum of the two products’ vibrational energies is less than the sum of their ZPEs, whereas the latter discards trajectories if either product has less vibrational energy than its ZPE.…”