It is critically important for many applications in chemistry to develop methods for accurate calculations of chemical reaction rates. For reactions occurring at sufficiently high temperatures and involving heavy atoms, classical molecular dynamics can be used to obtain accurate rate constants. At low temperatures, particularly for reactions involving the motion of light atoms, classical dynamics methods fail since quantum mechanical (QM) effects (such as zero point energy, tunneling and quantum coherence) become critically important. However, it is still impossible to solve the quantum reactive scattering problem for more than a few atoms. During the last two decades, a number of approximate quantum mechanical methods, which avoid the complications of the fully quantum mechanical formulation, have been proposed. One of such approximate methods - the ring polymer molecular dynamics (RPMD) rate theory - was recently developed at Oxford University. It has been shown that the well-known isomorphism between the quantum statistical mechanics of distinguishable particles and the classical statistical mechanics of harmonic ring polymers could be used as an approximation to the exact quantum dynamics of the system. The resulting ring polymer molecular dynamics (RPMD) method has been applied to a number of simple systems. We have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom chemical reactions.
We showed that this methodology can be extended to treat more complex polyatomic reactions in their full dimensionality, such as hydrogen abstraction reaction from methane, H + CH4→H2 + CH3. The results suggest that RPMD is one of the most promising methods for evaluating the rate constants for complex chemical reactions. The RPMD methodology has also been used to perform theoretical calculations of the rate coefficient for the Mu + H2 reaction over a wide range of temperatures. It was found that the RPMD approximation gives highly accurate results for the thermal rate coefficient of the reaction, in excellent agreement with exact QM calculations even at very low temperatures. The dominant QM effect in the reaction is the change in ZPE between the reactants and the products. The numerical evidence indicates that this effect is captured perfectly by RPMD.
RPMDrate is a free, open-source software package for using ring polymer molecular dynamics (RPMD) simulations to compute the bimolecular reaction rate coefficients for thermally activated processes in the gas phase.
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