Quantum Monte Carlo calculations in solids with downfolded Hamiltonians

TL;DR

This paper introduces a systematic downfolding approach combined with AFQMC to perform accurate, cost-effective many-body calculations on solids, reducing pseudopotential errors and enabling studies of complex materials.

Contribution

The authors develop a systematic downfolding method for extended systems that simplifies Hamiltonians while retaining material-specific properties, improving accuracy and efficiency in many-body calculations.

Findings

Achieved high accuracy for various solids including semiconductors, insulators, and metals.

Reduced computational cost significantly without losing accuracy.

Successfully determined the spin gap in NiO, demonstrating the method's effectiveness in strongly correlated materials.

Abstract

We present a systematic downfolding many-body approach for extended systems. Many-body calculations operate on a simpler Hamiltonian which retains material-specific properties. The Hamiltonian is systematically improvable and allows one to dial, in principle, between the simplest model and the original Hamiltonian. As a by-product, pseudopotential errors are essentially eliminated using a frozen-core treatment. The computational cost of the many-body calculation is dramatically reduced without sacrificing accuracy. We use the auxiliary-field quantum Monte Carlo (AFQMC) method to solve the downfolded Hamiltonian. Excellent accuracy is achieved for a range of solids, including semiconductors, ionic insulators, and metals. We further test the method by determining the spin gap in NiO, a challenging prototypical material with strong electron correlation effects. This approach greatly extends the reach of general, ab initio many-body calculations in materials.