Testing the Green's function coupled cluster singles and doubles impurity solver on real materials within the framework of self-energy embedding theory

We apply the Green's function coupled cluster singles and doubles (GFCCSD) impurity solver to realistic impurity problems arising for strongly correlated solids within the self-energy embedding theory (SEET) framework. We describe the details of our GFCC solver implementation, investigate its performance, and highlight potential advantages and problems on examples of impurities created during the self-consistent SEET for antiferromagnetic MnO and paramagnetic SrMnO3. GFCCSD provides satisfactory descriptions for weakly and moderately correlated impurities with sizes that are intractable by existing accurate impurity solvers such as exact diagonalization. However, our data also shows that when correlations become strong, the singles and doubles approximation used in GFCC could lead to instabilities in searching for the particle number present in impurity problems. These instabilities appear especially severe when the impurity size gets larger and multiple degenerate orbitals with strong correlations are present. We conclude that, to fully check the reliability of GFCCSD results and use them in fully ab initio calculations in the absence of experiments, a verification from a GFCC solver with higher order excitations is necessary.

Automated summary

The GFCCSD method provides satisfactory descriptions for weakly and moderately correlated impurities of large sizes, but instability in searching for particle numbers arises when dealing with strongly correlated impurities, especially with larger sizes and multiple degenerate orbitals with strong correlations. Verification from a GFCC solver with higher order excitations is necessary for fully checking the reliability of GFCCSD results.