Transcorrelated Methods for Multireference Problems

TL;DR

This paper introduces a workflow combining multireference wavefunctions with transcorrelated methods and FCIQMC to achieve highly accurate energies and excitation results for molecules, demonstrating improved accuracy over traditional approaches.

Contribution

It proposes a novel workflow integrating multireference wavefunctions with transcorrelated FCIQMC for improved multireference problem solutions.

Findings

Achieved chemical accuracy for N₂ binding curve.

Obtained accurate excitation energies for N₂, CO, and NH₃.

Demonstrated effectiveness with modest basis sets.

Abstract

We apply the transcorrelated method to problems of multireference character. For this, we show that the choice of reference wavefunction during the Jastrow optimisation procedure is vital, and we propose a workflow wherein we use conventional multi-configurational methods to provide a reference wavefunction for Jastrow factor optimisation. This Jastrow function is subsequently used with transcorrelated-full configuration interaction quantum Monte Carlo within the xTC approximation (TC-FCIQMC) to yield highly accurate transcorrelated energies. This is demonstrated for N$_2$ using the aug-cc-pVTZ basis set, achieving chemical accuracy across the entire binding curve compared with experiment. We also apply the method to compute excitation energies of dinitrogen, CO and the ammonia molecule, where accurate results, comparable to the best available theoretical predictions, are obtained with modest basis sets.