Constrained-Path Auxiliary-Field Quantum Monte Carlo for Coupled Electrons and Phonons

TL;DR

This paper introduces an extension of the CP-AFQMC method to simulate coupled electron-phonon systems, demonstrating high accuracy and improved autocorrelation scaling, useful for complex materials and lattice models.

Contribution

The authors develop a novel combined quantum Monte Carlo approach for electrons and phonons, enhancing accuracy and efficiency in modeling electron-phonon interactions.

Findings

Accurate results for Holstein models with simple trial wavefunctions.

Autocorrelation time scales as 1/ω, better than traditional methods.

Method performs well in Hubbard-Holstein models, especially when electron-phonon coupling dominates.

Abstract

We present an extension of constrained-path auxiliary-field quantum Monte Carlo (CP-AFQMC) for the treatment of correlated electronic systems coupled to phonons. The algorithm follows the standard CP-AFQMC approach for description of the electronic degrees of freedom while phonons are described in first quantization and propagated via a diffusion Monte Carlo approach. Our method is tested on the one- and two-dimensional Holstein and Hubbard-Holstein models. With a simple semiclassical trial wavefunction, our approach is remarkably accurate for $\omega/(2\text{d}t\lambda) < 1$ for all parameters in the Holstein model considered in this study. In addition, we empirically show that the autocorrelation time scales as $1/\omega$ for $\omega/t \lesssim 1$, which is an improvement over the $1/\omega^2$ scaling of the conventional determinant quantum Monte Carlo algorithm. In the Hubbard-Holstein model, the accuracy of our algorithm is found to be consistent with that of standard CP-AFQMC for the Hubbard model when the Hubbard $U$ term dominates the physics of the model, and is nearly exact when the ground state is dominated by the electron-phonon coupling scale $\lambda$. The approach developed in this work should be valuable for understanding the complex physics arising from the interplay between electrons and phonons in both model lattice problems and ab-initio systems.