Improved Stochastic Multireference Perturbation Theory for Correlated Systems with Large Active Spaces

TL;DR

This paper improves the efficiency of stochastic NEVPT2 calculations for large active spaces by introducing an additional stochastic sampling step, significantly reducing computational time while maintaining accuracy, enabling routine calculations for complex correlated systems.

Contribution

It introduces an effective stochastic sampling enhancement within FCIQMC-NEVPT2, reducing computational cost and enabling routine large active space calculations for strongly correlated molecules.

Findings

Achieved up to 80% reduction in simulation time.

Successfully applied to spin states of an iron porphyrin system.

Enabled routine NEVPT2 calculations for (24,24) active spaces.

Abstract

We identify the dominant computational cost within the recently introduced stochastic and internally contracted FCIQMC-NEVPT2 method for large active space sizes. This arises from the contribution to the four-body intermediates arising from low-excitation level sampled determinant pairs. We develop an effective way to mitigate this cost via an additional stochastic step within the sampling of the required NEVPT2 intermediates. We find this systematically improvable additional sampling can reduce simulation time by 80\% without introducing appreciable error. This saving is expected to increase for larger active spaces. We combine this enhanced sampling scheme with full stochastic orbital optimization for the first time, and apply it to find FCIQMC-NEVPT2 energies for spin states of an iron porphyrin system within (24,24) active spaces with relatively meagre computational resources. This active space size can now be considered as routine for NEVPT2 calculations of strongly correlated molecular systems within this improved stochastic methodology.