Triple excitations in Green's function coupled cluster solver for studies of strongly correlated systems in the framework of self-energy embedding theory

TL;DR

This paper introduces a Green's function coupled cluster solver with singles, doubles, and triples (GFCCSDT) for accurate, systematic studies of strongly correlated molecules and solids within embedding theories, enabling treatment of larger orbital spaces.

Contribution

The development of a GFCCSDT solver capable of handling large orbital spaces in molecules and solids, improving accuracy and systematic convergence in Green's function embedding calculations.

Findings

GFCCSDT accurately describes strongly correlated systems like SrMnO3.

An approximate GFCCSDT variant yields high-accuracy spectral functions for MnO.

The solver enables systematic convergence testing in computational studies.

Abstract

Embedding theories became important approaches used for accurate calculations of both molecules and solids. In these theories, a small chosen subset of orbitals is treated with an accurate method, called an impurity solver, capable of describing higher correlation effects. Ideally, such a chosen fragment should contain multiple orbitals responsible for the chemical and physical behavior of the compound. Handing a large number of chosen orbitals presents a very significant challenge for the current generation of solvers used in the physics and chemistry community. Here, we develop a Green's function coupled cluster singles doubles and triples (GFCCSDT) solver that can be used for a quantitative description in both molecules and solids. This solver allows us to treat orbital spaces that are inaccessible to other accurate solvers. At the same time, GFCCSDT maintains high accuracy of the resulting self-energy. Moreover, in conjunction with the GFCCSD solver, it allows us to test the systematic convergence of computational studies. Developing the CC family of solvers paves the road to fully systematic Green's function embedding calculations in solids. In this paper, we focus on the investigation of GFCCSDT self-energies for a strongly correlated problem of SrMnO$_3$ solid. Subsequently, we apply this solver to solid MnO showing that an approximate variant of GFCCSDT is capable of yielding a high accuracy orbital resolved spectral function.