Efficient Modelling of Anharmonicity and Quantum Effects in PdCuH$_2$ with Machine Learning Potentials

TL;DR

This paper introduces a machine learning-based protocol combined with SSCHA to efficiently model quantum effects and anharmonicity in materials, demonstrated on PdCuH$_x$ compounds, significantly reducing computational costs.

Contribution

The authors develop a new method pairing machine learning potentials with SSCHA, enabling large-scale, cost-effective quantum and anharmonicity modeling in complex materials.

Findings

Identified a stable PdCuH$_2$ structure relevant to superconductivity.

Reduced SSCHA computational expense by approximately 96%.

Demonstrated the method's potential for routine quantum effects inclusion in materials discovery.

Abstract

Quantum nuclear effects and anharmonicity impact a wide range of functional materials and their properties. One of the most powerful techniques to model these effects is the Stochastic Self-Consistent Harmonic Approximation (SSCHA). Unfortunately, the SSCHA is extremely computationally expensive, prohibiting its routine use. We propose a protocol that pairs machine learning interatomic potentials, which can be tailored for the system at hand via active learning, with the SSCHA. Our method leverages an upscaling procedure that allows for the treatment of supercells of up to thousands of atoms with practically minimal computational effort. The protocol is applied to PdCuH$_x$ ($x = 0-2$) compounds, chosen because previous experimental studies have reported superconducting critical temperatures, $T_\text{c}$s, as high as 17~K at ambient pressures in an unknown hydrogenated PdCu phase. We identify a $P4/mmm$ PdCuH$_2$ structure, which is shown to be dynamically stable only upon the inclusion of quantum fluctuations, as being a key contributor to the measured superconductivity. For this system, our methodology is able to reduce the computational expense for the SSCHA calculations by $\sim$96\%. The proposed protocol opens the door towards the routine inclusion of quantum nuclear motion and anharmonicity in materials discovery.