Developments in Stochastic Coupled Cluster Theory: The initiator approximation and application to the Uniform Electron Gas

TL;DR

This paper advances stochastic coupled cluster theory by introducing an initiator approximation, enabling accurate energy calculations for the uniform electron gas at high densities, including previously unattainable CCSDT energies.

Contribution

It presents an initiator modification to stochastic coupled cluster theory and demonstrates its application to the uniform electron gas, achieving high-accuracy correlation energies.

Findings

Initiator stochastic coupled cluster can be extrapolated to the unbiased limit.

Complete basis set limit CCSD and CCSDT energies are obtained for the uniform electron gas.

Triple excitations are necessary for accurate energies at high densities.

Abstract

We describe further details of the Stochastic Coupled Cluster method and a diagnostic of such calculations, the shoulder height, akin to the plateau found in Full Configuration Interaction Quantum Monte Carlo. We describe an initiator modification to Stochastic Coupled Cluster Theory and show that initiator calculations can be extrapolated to the unbiased limit. We apply this method to the 3D 14-electron uniform electron gas and present complete basis set limit values of the CCSD and previously unattainable CCSDT correlation energies for up to rs = 2, showing a requirement to include triple excitations to accurately calculate energies at high densities.