Converged Colored Noise Path Integral Molecular Dynamics Study of the Zundel Cation down to Ultra-low Temperatures at Coupled Cluster Accuracy

TL;DR

This study demonstrates that advanced colored noise thermostatting schemes enable accurate and efficient path integral simulations of hydrogen-bonded systems like the Zundel cation at ultra-low temperatures, achieving near-Coupled Cluster accuracy.

Contribution

It compares the effectiveness of PIGLET and PIQTB thermostat schemes in simulating hydrogen bonds at Kelvin temperatures with high accuracy using neural network potentials.

Findings

Both thermostat schemes accurately reproduce quantum delocalization effects.

The methods are computationally feasible for ultra-low temperature simulations.

Results validate the use of these schemes for hydrogen-bonded systems at cryogenic temperatures.

Abstract

For a long time, performing converged path integral simulations at ultra-low, but finite temperatures of a few Kelvin has been a nearly impossible task. However, recent developments in advanced colored noise thermostatting schemes for path integral simulations, namely the Path Integral Generalized Langevin Equation Thermostat (PIGLET) and the Path Integral Quantum Thermal Bath (PIQTB), have been able to greatly reduce the computational cost of these simulations, thus making the ultra-low temperature regime accessible in practice. In this work, we investigate the influence of these two thermostatting schemes on the description of hydrogen-bonded systems at temperatures down to a few Kelvin as encountered, for example, in helium nanodroplet isolation or tagging photodissociation spectroscopy experiments. For this purpose, we analyze the prototypical hydrogen bond in the Zundel cation (H$_5$O$_{2}^{+}$) as a function of both, oxygen-oxygen distance and temperature in order to elucidate how the anisotropic quantum delocalization and, thus, the shape of the shared proton adapts depending on the donor-acceptor distance. The underlying electronic structure of the Zundel cation is described in terms of Behler's Neural Network Potentials of essentially converged Coupled Cluster accuracy, CCSD(T*)-F12a/AVTZ. In addition, the performances of the PIQTB and PIGLET methods for energetic, structural, and quantum delocalization properties are assessed and directly compared. Overall, our results emphasize the validity and practical usefulness of these two modern thermostatting approaches for path integral simulations of hydrogen-bonded systems even at ultra-low temperatures.