Capturing spin fluctuations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CaCuO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>: <i>Ab initio</i> quantum Monte Carlo calculations with multideterminant wave functions

We present an advanced ab initio quantum Monte Carlo (QMC) calculation of the ground state of undoped ${\mathrm{CaCuO}}_{2}$. We extend the traditional single-determinant Slater-Jastrow approach to include multideterminant 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 multideterminant wave functions in a strongly correlated cuprate system, which is integral to understanding high-${T}_{c}$ superconductivity. This is a demonstration of capturing spin fluctuations in QMC wave functions on a cuprate, establishing the groundwork for studies on doped cuprates in the superconducting state, where spin fluctuations require more accurate characterization.