Stretching bonds in Density Functional Theory without artificial symmetry breaking

TL;DR

This paper introduces an embedding framework using fragment spin densities to address the symmetry dilemma in density functional theory, improving energy calculations for stretched molecules without artificial symmetry breaking.

Contribution

It proposes a novel overlap approximation based on fragment densities that enhances the accuracy of DFT calculations for stretched molecules, avoiding symmetry breaking.

Findings

Significant improvement in binding energy predictions.

Elimination of artificial symmetry breaking in stretched molecules.

Effective use of fragment densities in DFT calculations.

Abstract

Accurate first-principles calculations for the energies, charge distributions, and spin symmetries of many-electron systems are essential to understand and predict the electronic and structural properties of molecules and materials. Kohn-Sham density functional theory (KS-DFT) stands out among electronic-structure methods due to its balance of accuracy and computational efficiency. It is now extensively used in fields ranging from materials engineering to rational drug design. However, to achieve chemically accurate energies, standard density functional approximations in KS-DFT often need to break underlying symmetries, a long-standing "symmetry dilemma". By employing fragment spin densities as the main variables in calculations (rather than total molecular densities as in KS-DFT), we present an embedding framework in which this symmetry dilemma is resolved for the case of stretched molecules. The spatial overlap between fragment densities is used as the main ingredient to construct a simple, physically-motivated approximation to a universal functional of the fragment densities. This 'overlap approximation' is shown to significantly improve semi-local KS-DFT binding energies of molecules without artificial symmetry breaking.