An explicitly correlated approach to basis set incompleteness in Full Configuration Interaction Quantum Monte Carlo

TL;DR

This paper introduces an explicitly correlated FCIQMC method that significantly improves basis set convergence and accuracy in quantum chemistry calculations by coupling FCIQMC with R12 techniques.

Contribution

It develops a new FCIQMC-F12 approach that efficiently incorporates explicit correlation, reducing basis set errors and enhancing accuracy in strongly correlated systems.

Findings

F12 corrections rapidly converge with sampling

Improved accuracy over previous FCIQMC methods

Achieved basis set quality equivalent to two cardinal number increase

Abstract

By performing a stochastic dynamic in a space of Slater determinants, the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) method has been able to obtain energies which are essentially free from systematic error to the basis set correlation energy, within small and systematically improvable errorbars. However, the weakly exponential scaling with basis size makes converging the energy with respect to basis set costly and in larger systems, impossible. To ameliorate these basis set issues, here we use perturbation theory to couple the FCIQMC wave function to an explicitly correlated strongly orthogonal basis of geminals, following the [2]_{\textrm{R12}} approach of Valeev {\em et al.}. The required one- and two-particle density matrices are computed on-the-fly during the FCIQMC dynamic, using a sampling procedure which incurs relatively little additional computation expense. The F12 energy corrections are shown to converge rapidly as a function of sampling, both in imaginary time, and number of walkers. Our pilot calculations on the binding curve for carbon dimer, which exhibits strong correlation effects as well as substantial basis set dependence, demonstrate that the accuracy of the FCIQMC-F12 method surpasses that of all previous FCIQMC calculations, and that the F12 correction improves accuracy equivalent to increasing the quality of the one-electron basis by two cardinal numbers.