Eliminating spin contamination in auxiliary-field quantum Monte Carlo: realistic potential energy curve of F2

TL;DR

This paper introduces a spin-projection method for auxiliary-field quantum Monte Carlo (AFQMC) that removes spin contamination, enabling accurate potential energy curves for challenging molecules like F2, even with spin-contaminated reference states.

Contribution

A simple spin-projection technique is developed for AFQMC, improving accuracy and efficiency in systems with spin contamination, demonstrated on the F2 molecule.

Findings

Eliminates spin contamination in AFQMC calculations.

Produces accurate potential energy curves for F2 from equilibrium to dissociation.

Spectroscopic constants agree well with experimental data.

Abstract

The use of an approximate reference state wave function |Phi_r> in electronic many-body methods can break the spin symmetry of Born-Oppenheimer spin-independent Hamiltonians. This can result in significant errors, especially when bonds are stretched or broken. A simple spin-projection method is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations, which yields spin-contamination-free results, even with a spin-contaminated |Phi_r>. The method is applied to the difficult F2 molecule, which is unbound within unrestricted Hartree-Fock (UHF). With a UHF |Phi_r>, spin contamination causes large systematic errors and long equilibration times in AFQMC in the intermediate, bond-breaking region. The spin-projection method eliminates these problems, and delivers an accurate potential energy curve from equilibrium to the dissociation limit using the UHF |Phi_r>. Realistic potential energy curves are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are in excellent agreement with experiment.