Quantum instanton approximation for thermal rate constants of chemical reactions

  • Miller W
  • Zhao Y
  • Ceotto M
 et al. 
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A quantum mechanical theory for chemical reaction rates is presented which is modeled after the semiclassical SC instanton approximation. It incorporates the desirable aspects of the instanton picture, which involves only properties of the SC approximation to the Boltzmann operator, but corrects its quantitative deficiencies by replacing the SC approximation for the Boltzmann operator by the quantum Boltzmann operator, exp( ˆ ). Since a calculation of the quantum Boltzmann H operator is feasible for quite complex molecular systems by Monte Carlo path integral methods, having an accurate rate theory that involves only the Boltzmann operator could be quite useful. The application of this quantum instanton approximation to several one- and two-dimensional model problems illustrates its potential; e.g., it is able to describe thermal rate constants accurately (10–20% error from high to low temperatures deep in the tunneling regime, and applies equally well to asymmetric and symmetric potentials. © 2003 American Institute of Physics. DOI: 10.1063/1.1580110

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