Ab initio electron-lattice downfolding: potential energy landscapes, anharmonicity, and molecular dynamics in charge density wave materials

TL;DR

This paper introduces downfolding methods to simplify electronic structure calculations, enabling efficient molecular dynamics simulations of charge density wave materials while accurately capturing potential energy landscapes and anharmonic effects.

Contribution

It develops and benchmarks three downfolding strategies that significantly accelerate MD simulations of CDW materials with high accuracy compared to full DFT calculations.

Findings

Downfolded models reproduce potential energy surfaces accurately.

Achieve about five orders of magnitude speedup in MD simulations.

Thermal and quantum fluctuations influence CDW transitions in monolayer 1H-TaS$_2$.

Abstract

The interplay of electronic and nuclear degrees of freedom presents an outstanding problem in condensed matter physics and chemistry. Computational challenges arise especially for large systems, long time scales, in nonequilibrium, or in systems with strong correlations. In this work, we show how downfolding approaches facilitate complexity reduction on the electronic side and thereby boost the simulation of electronic properties and nuclear motion - in particular molecular dynamics (MD) simulations. Three different downfolding strategies based on constraining, unscreening, and combinations thereof are benchmarked against full density functional calculations for selected charge density wave (CDW) systems, namely 1H-TaS$_2$, 1T-TiSe$_2$, 1H-NbS$_2$, and a one-dimensional carbon chain. We find that the downfolded models can reproduce potential energy surfaces on supercells accurately and facilitate computational speedup in MD simulations by about five orders of magnitude in comparison to purely ab initio calculations. For monolayer 1H-TaS$_2$ we report classical replica exchange and quantum path integral MD simulations, revealing the impact of thermal and quantum fluctuations on the CDW transition.