Quantum and Semiclassical Methods for Molecular Rate Constants and Vibrational Spectra Calculations

This presentation reports about a quantum approximation for thermal rate constant calculations. The approximate rate expression is obtained from a stationary phase approximation to the time integral of the flux-flux correlation function. The resulting expression is shown to barely depend on the position of the flux operators, i.e. of the dividing surfaces. This property opens the route to applications to complex systems. We show how the approximation works on one and two dimensional systems with predominant quantum effects over a wide range of temperatures and the results are within few percent of the exact values, for a reasonable range of dividing surface positions.[1] In a second part of the presentation, we show an optimized approach for the calculation of the vibrational density of states and the thermal rate constants in high-dimensional systems. We introduce a new code, called ParAdensum, which is based on the implementation of the Wang−Landau Monte Carlo algorithm for parallel architectures and part of the MULTIWELL suite.[2] We test the accuracy of ParAdensum on several molecular systems and show a significant computational speed-up with respect to standard approaches. The new code can easily handle 150 degrees of freedom.[3] In a third part, a new semiclassical “divide-and-conquer” approach is presented. Here, the goal is to demonstrate that semiclassical dynamics simulations of high dimensional real molecular systems are doable. We first show the calculation of the quantum vibrational power spectra of small molecules for which benchmark quantum results are available, and we calculate the spectrum of C60, a system characterized by 174 vibrational degrees of freedom. The results show that quantum anharmonicities and purely quantum features like overtones are accurately accounted for, including when the molecular symmetry is broken and the degeneracy removed.[4] References [1] C. Aieta, M. Ceotto, submitted to J. Chem Phys. (under revision) [2] J. R. Barker, et al. MultiWell Program Suite, Version 2014.1b; University of Michigan: Ann Arbor, MI, 2014; http://aoss-research.engin.umich.edu/multiwell/ (accessed June 25, 2014). [3] C. Aieta, F. Gabas, and M. Ceotto, J. Phys. Chem. A 120, 4853 (2016) [4] M. Ceotto, G. Di Liberto, and R. Conte, submitted to Phys. Rev. Lett. (under revision)