Quantum rates in dissipative systems with spatially varying friction

TL;DR

This study explores how spatially varying friction influences quantum reaction rates in dissipative systems, revealing complex quantum-classical differences and identifying regimes dominated by tunneling effects across temperature ranges.

Contribution

It introduces a detailed analysis of quantum effects in reaction rates with spatially dependent friction, using advanced simulation methods to identify different dynamical regimes.

Findings

Quantum rates differ significantly from classical predictions.

RPMD effectively captures most quantum effects in reaction rates.

Quantum tunneling dominates at low temperatures and low friction.

Abstract

We investigate whether making the friction spatially dependent on the reaction coordinate introduces quantum effects into the thermal reaction rates for dissipative reactions. Quantum rates are calculated using the numerically exact multi-configuration time-dependent Hartree (MCTDH) method, as well as the approximate ring-polymer molecular dynamics (RPMD), ring-polymer instanton (RPI) methods, and classical mechanics. By conducting simulations across a wide range of temperatures and friction strengths, we can identify the various regimes that govern the reactive dynamics. At high temperatures, in addition to the spatial-diffusion and energy-diffusion regimes predicted by Kramer's rate theory, a (coherent) tunnelling-dominated regime is identified at low friction. At low temperatures, incoherent tunnelling dominates most of Kramer's curve, except at very low friction when coherent tunnelling becomes dominant. Unlike in classical mechanics, the bath's influence changes the equilibrium time-independent properties of the system, leading to a complex interplay between spatially dependent friction and nuclear quantum effects even at high temperatures. More specifically, a realistic friction profile can lead to an increase (decrease) of the quantum (classical) rates with friction within the spatial-diffusion regime, showing that classical and quantum rates display qualitatively different behaviours. Except at very low frictions, we find that RPMD captures most of the quantum effects in the thermal reaction rates.