Coupled Cluster Greens function formulations based on the effective Hamiltonians

TL;DR

This paper introduces a combined DUCC-GFCC method that efficiently computes molecular spectral functions by using effective Hamiltonians, reducing computational effort while maintaining accuracy in selected energy ranges.

Contribution

The novel integration of DUCC-derived effective Hamiltonians with GFCC formalism enhances computational efficiency for spectral calculations in molecular systems.

Findings

Good agreement with all-orbital GFCC methods

Significant reduction in numerical effort

Effective in energy windows aligned with active space

Abstract

We demonstrate that the effective Hamiltonians obtained with the downfolding procedure based on double unitary coupled cluster (DUCC) ansatz can be used in the context of Greens function coupled cluster (GFCC) formalism to calculate spectral functions of molecular systems. This combined approach (DUCC-GFCC) provides a significant reduction of numerical effort and good agreement with the corresponding all-orbital GFCC methods in energy windows that are consistent with the choice of active space. These features are demonstrated on the example of two benchmark systems: H2O and N2, where DUCC-GFCC calculations were performed for active spaces of various sizes.