Towards ab initio self-energy embedding theory in quantum chemistry

TL;DR

This paper extends the self-energy embedding theory (SEET) to ab initio quantum chemistry, demonstrating its effectiveness in accurately describing molecular systems with large active spaces and avoiding common computational issues.

Contribution

The paper introduces an extension of SEET to quantum chemical Hamiltonians, enabling accurate and efficient treatment of strong correlations in molecules with large active spaces.

Findings

SEET yields results comparable to NEVPT2 for small molecules.

SEET can split large active spaces into smaller ones without additional implementation.

SEET avoids intruder states and high-order RDMs, simplifying calculations.

Abstract

The self-energy embedding theory (SEET), in which the active space self-energy is embedded in the self-energy obtained from a perturbative method treating the non-local correlation effects, was recently developed in our group. In SEET the double counting problem does not appear and the accuracy can be improved either by increasing the perturbation order or by enlarging the active space. This method was first calibrated for the 2D Hubbard lattice showing promising results. In this paper, we report an extension of SEET to quantum chemical ab initio Hamiltonians for applications to molecular systems. The self-consistent second-order Green's function (GF2) method is used to describe the non-local correlations, while the full configuration interaction (FCI) method is carried out to capture strong correlation within the active space. Using few proof-of-concept examples, we show that SEET yields results of comparable quality to $n-$electron valence state second-order perturbation theory (NEVPT2) with the same active space, and furthermore, the full active space can be split into smaller active spaces without further implementation. Moreover, SEET avoids intruder states and does not require any high-order reduced density matrices. These advantages show that SEET is a promising method to describe physical and chemical properties of challenging molecules requiring large active spaces.