Guided diffusion monte carlo based on bonding environment: an efficient approach for studying molecular vibrations in pathological systems

Finney, Jacob M.

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https://hdl.handle.net/2142/107657 Copy

Description

Title

Guided diffusion monte carlo based on bonding environment: an efficient approach for studying molecular vibrations in pathological systems

Author(s)

Finney, Jacob M.

Contributor(s)

McCoy, Anne B.

Issue Date

2020-06-22

Keyword(s)

Theory and Computation

Abstract

\par Diffusion Monte Carlo (DMC) is a technique that can be used to obtain the ground state energy and ground state wave function given a potential energy surface (PES) that fully describes the system of interest. However, one complication with this technique is that in order to obtain accurate results for molecular systems that have couplings between the high and low frequency vibrations, large ensemble sizes are needed. \footnote{Mallory, J. D. and Mandelshtam, V. A., {\it{J. Phys. Chem. A}} (2015), {\bf{119}}, 6504-6515.} One approach to combat these large ensemble sizes is to use partial importance sampling to describe the high-frequency vibrations. This approach has been applied to studies of neutral water clusters, where a significant reduction in the ensemble sizes that were needed in the simulations was achieved. \footnote{Lee, V. G. M. and McCoy, A.B., {\it{J. Phys. Chem. A}} (2019), {\bf{123}}, 37, 8063-8070.} \par In this talk, I will describe the applications of this approach through the study of two systems, where the high-frequency vibrations are highly sensitive to the value of the coordinates associated with the low-frequency vibrations. The first system is \chem{CH_5^+}, where the equilibrium CH bond lengths vary from 1.09 to 1.20 \AA in the low energy stationary points on the PES. In the ground state, each CH oscillator samples all these positions leading to a dependence of the CH frequency on the coordinates that are associated with the large amplitude vibrations that connect these minima. The second system of interest is protonated water clusters where the high-frequency shared proton stretch frequency varies between 1000 to 3000 \wn depending on its environment. By using correlations between the bond lengths and frequencies of the CH and OH oscillators and the structure of the ion, a flexible function can be constructed to approximate the effect that the environment has on these vibrations. By describing the CH and OH wavefunctions through importance sampling based on these flexible functions and allowing the other coordinates to be sampled using standard DMC techniques we can reduce the ensemble size needed to obtain converged zero-point energies and ground state wave functions. Extensions of these studies to larger systems, where these couplings between high and low frequency vibrations are prevalent, will also be discussed.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

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http://hdl.handle.net/2142/107657

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Copyright 2020 is held by the Author(s)

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