Abstract
We describe and test on some organic reactions a parallel implementation strategy to compute anharmonic constants, which are employed in semiclassical transition state theory reaction rate calculations. Our software can interface with any quantum chemistry code capable of a single point energy estimate, and it is suitable for both minimum and transition state geometry calculations. After testing the accuracy and comparing the efficiency of our implementation against other software, we use it to estimate the semiclassical transition state theory (SCTST) rate constant of three reactions of increasing dimensionality, known as examples of heavy atom tunneling. We show how our method is improved in efficiency with respect to other existing implementations. In conclusion, our approach allows SCTST rates and heavy atom tunneling at a high level of electronic structure theory (up to CCSD(T)) to be evaluated. This work shows how crucial the possibility to perform high level ab initio rate evaluations can be.
Highlights
Calculation of reaction rates in theoretical chemistry is still nowadays a challenging task
We use the anharmonic constants as input for the semiclassical transition state theory (SCTST) programs Paradensum and Parsctst[41,42] from the Multiwell program suite.[38]
We tested the performances of our algorithm, showing that it successfully overcomes the limitations of other software
Summary
Calculation of reaction rates in theoretical chemistry is still nowadays a challenging task. Transition state theory (TST) is a clever rate constant approximation that avoids dynamics simulations and delivers rates in terms of static thermodynamics information.[13,14] Since TST is a classical mechanics theory, early theories based on onedimensional potential approximation were elaborated to account for quantum tunneling, such as Wigner or Eckart corrections.[15,16] Nowadays, more sophisticated approximations have been developed to include, at least to some extent, the effects neglected by 1D approaches to tunneling corrections and limitations of the TST method itself, such as corner cutting, nonseparability of the reaction coordinate, and recrossing.[17−28]. Miller in the 1970s and revisited in the 1990s29−32 has received renewed attention in the past few years.[33−37] What makes SCTST convenient for application is that it requires input quantities that are routinely calculated by quantum chemistry codes These include the harmonic vibrational frequencies, the height of the reaction barrier, and the anharmonic vibrational coupling constants, which are employed in the context of second order vibrational perturbation theory (VPT2). In the Summary and Conclusions section (Section 5), we provide our final remarks and anticipate some future perspective
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