Parallel Implementation of Anharmonic Constants to Speedup Semiclassical Transition State Theory Rate Constant Calculations

We devised and tested 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 single point energy estimate and it is suitable for both minimum and transition state geometry calculations. After testing the accuracy and compare the efficiency of our implementation against available 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 to evaluate SCTST rates and heavy atom tunneling at high level of electronic structure theory (up to CCSD(T)). This work will show how crucial can be the possibility to perform high level ab initio rate evaluations.