title: Direct evaluation of the temperature dependence of the rate constant based on the quantum instanton approximation.
abstract: A general method for the direct evaluation of the temperature dependence of the quantum-mechanical reaction rate constant in many-dimensional systems is described. The method is based on the quantum instanton approximation for the rate constant, thermodynamic integration with respect to the inverse temperature, and the path integral Monte Carlo evaluation. It can describe deviations from the Arrhenius law due to the coupling of rotations and vibrations, zero-point energy, tunneling, corner-cutting, and other nuclear quantum effects. The method is tested on the Eckart barrier and the full-dimensional H+H(2)-->H(2)+H reaction. In the temperature range from 300 to 1500 K, the error of the present method remains within 13% despite the very large deviations from the Arrhenius law. The direct approach makes the calculations much more efficient, and the efficiency is increased even further (by up to two orders of magnitude in the studied reactions) by using optimal estimators for reactant and transition state thermal energies. Which of the estimators is optimal, however, depends on the system and the strength of constraint in a constrained simulation.
authors.affiliation: Laboratory of Theoretical Physical Chemistry, Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne CH-1015, Switzerland.
authors.forename: Marcin
authors.initials: M
authors.lastname: Buchowiecki
authors.forename: Jirí
authors.initials: J
authors.lastname: Vanícek
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D009010:Monte Carlo Method; D011789:Quantum Theory; D013696:Temperature; D013816:Thermodynamics
chemical_list: D006859:Hydrogen