Exploring Coupled Cluster Green's function as a method for treating system and environment in Green's function embedding methods

TL;DR

This paper evaluates the coupled cluster Green's function (GFCC) method within the self-energy embedding theory framework, analyzing its effectiveness as an impurity solver for system and environment treatment, and exploring how impurity size affects accuracy.

Contribution

It demonstrates the limitations of GFCC as an impurity solver for larger problems and suggests increasing the solver's rank and using natural orbitals for improved accuracy.

Findings

GFCC performs well for small impurities.

Performance deteriorates with larger impurity sizes.

Natural orbitals outperform symmetrized atomic orbitals for total energy calculations.

Abstract

Within the self-energy embedding theory (SEET) framework, we study coupled cluster Green's function (GFCC) method in two different contexts: as a method to treat either the system or environment present in the embedding construction. Our study reveals that when GFCC is used to treat the environment we do not see improvement in total energies in comparison to the coupled cluster method itself. To rationalize this puzzling result, we analyze the performance of GFCC as an impurity solver with a series of transition metal oxides. These studies shed light on strength and weaknesses of such a solver and demonstrate that such a solver gives very accurate results when the size of the impurity is small. We investigate if it is possible to achieve a systematic accuracy of the embedding solution when we increase the size of the impurity problem. We found that in such a case, the performance of the solver worsens, both in terms of finding the ground state solution of the impurity problem as well as the self-energies produced. We concluded that increasing the rank of GFCC solver is necessary to be able to enlarge impurity problems and achieve a reliable accuracy. We also have shown that natural orbitals from weakly correlated perturbative methods are better suited than symmetrized atomic orbitals (SAO) when the total energy of the system is the target quantity.