# Full wave function optimization with quantum Monte Carlo and its effect on the dissociation energy of FeS

## TL;DR

This study employs advanced quantum Monte Carlo methods with orbital and guide function optimization to accurately predict the dissociation energy of FeS, revealing the importance of orbital optimization and Jastrow factors in transition metal compounds.

## Contribution

First implementation of quantum Monte Carlo orbital optimization for transition metal molecules, improving agreement with experimental dissociation energies.

## Key findings

- Orbital optimization yields the correct $^5 riangle$ ground state.
- Jastrow factor size significantly affects DMC accuracy.
- Optimized orbitals are essential for accurate energy predictions.

## Abstract

Diffusion quantum Monte Carlo calculations with partial and full optimization of the guide function are carried out for the dissociation of the FeS molecule. For the first time, quantum Monte Carlo orbital optimization for transition metal compounds is performed. It is demonstrated that energy optimization of the orbitals of a complete active space wave function in the presence of a Jastrow correlation function is required to obtain agreement with the experimental dissociation energy. Furthermore, it is shown that orbital optimization leads to a $^5\Delta$ ground state, in agreement with experiments, but in disagreement with other high-level ab initio wave function calculations which all predict a $^5\Sigma^+$ ground state. The role of the Jastrow factor in DMC calculations with pseudo potentials is investigated. The results suggest that a large Jastrow factor may improve the DMC accuracy substantially at small additional cost.