Towards a fully size-consistent method of increments

TL;DR

This paper introduces a size-consistent multiconfigurational method of increments (MoI) that improves the calculation of dissociation curves by addressing size-inconsistency issues inherent in the standard approach, showing promising results against DMRG benchmarks.

Contribution

It develops a size-consistent multiconfigurational MoI approach using a two-state constant-coupling scheme for better dissociation curve calculations.

Findings

Ground state results are very promising across the dissociation curve.

Excited state energies are accurate near avoided crossings.

The method shows good agreement with DMRG benchmarks.

Abstract

The method of increments (MoI) allows one to successfully calculate cohesive energies of bulk materials with high accuracy, but it encounters difficulties when calculating whole dissociation curves. The reason is that its standard formalism is based on a single Hartree-Fock (HF) configuration whose orbitals are localized and used for the many-body expansion. Therefore, in those situations where HF does not allow a size-consistent description of the dissociation, the MoI cannot yield proper results either. Herein we address the problem by employing a size-consistent multiconfigurational reference for the MoI formalism. This leads to a matrix equation where a coupling derived by the reference itself is employed. In principle, such approach allows one to evaluate approximate values for the ground as well as excited states energies. While the latter are accurate close to the avoided crossing only, the ground state results are very promising for the whole dissociation curve, as shown by the comparison with density matrix renormalization group (DMRG) benchmarks. We tested this two-state constant-coupling (TSCC)-MoI on beryllium rings of different sizes and studied the error introduced by the constant coupling.