Capturing spin fluctuations in CaCuO$_2$: $\textit{Ab initio}$ QMC calculations with multi-determinant wave functions

TL;DR

This paper advances ab initio quantum Monte Carlo methods by incorporating multi-determinant wave functions to better capture spin fluctuations in CaCuO$_2$, a key to understanding high-temperature superconductivity.

Contribution

It introduces multi-determinant wave functions and orbital optimization in QMC for cuprates, improving energy bounds and capturing spin fluctuations.

Findings

Ground state energy lowered by 2.3 eV per formula unit.

First demonstration of spin fluctuations in QMC wave functions for cuprates.

Establishes groundwork for studying doped cuprates in superconducting state.

Abstract

We present an advanced $\textit{ab initio}$ quantum Monte Carlo (QMC) calculation of the ground state of undoped CaCuO$_2$. We extend the traditional single-determinant Slater-Jastrow approach to include multi-determinant wave functions, inhomogeneous Jastrow factors, and orbital optimization. Our results demonstrate not only an improvement in the variational bound of the ground state energy -- 2.3 eV per formula unit lower than previous state of the art techniques -- but also confirm the presence of spin fluctuations in multi-determinant wave functions in a strongly correlated cuprate system, which is integral to understanding high-$T_c$ superconductivity. This is the first demonstration of capturing spin fluctuations in QMC wave functions on a cuprate, establishing the groundwork for new studies on doped cuprates in the superconducting state, where spin fluctuations require more accurate characterization.