Abstract

A comprehensive description of nuclei dynamics is crucial for accurately modeling processes in condensed matter physics, spanning from thermodynamic and structural properties to transport and superconductivity. While the harmonic approximation of ionic fluctuations (noninteracting phonons), with nuclei treated as classical particles, serves as the fundamental approach to tackle the nuclei dynamics problem from first principles, its validity is significantly compromised near structural phase transitions or when ions undergo large oscillations. In such scenarios, perturbative approaches to include anharmonic effects are ruled out. Notably, this challenge arises prominently in materials containing hydrogen, where the substantial zero-point motion of protons leads to a breakdown of the harmonic approximation, even at low temperatures. To address the incorporation of quantum and anharmonic effects in nuclei dynamics at non-perturbative level, we introduce a variational method tailored for this purpose, known as the stochastic self-consistent harmonic approximation (SSCHA) [1]. In this presentation, I will outline this method and showcase examples demonstrating how its application can overcome the limitations of standard approaches.

[1] L. Monacelli, R. Bianco, L. Cherubini, M. Calandra, I. Errea, and F. Mauri, J. Phys.: Condens. Matter 33, 363001 (2021)