One-electron self-interaction error and its relationship to geometry and higher orbital occupation

TL;DR

This paper analyzes the one-electron self-interaction error in DFT, revealing how molecular shape, geometry, and orbital occupation influence the error, and highlights the need for more robust density functional approximations.

Contribution

It provides a detailed visualization and analysis of the one-electron SIE across various geometries and orbital occupations, offering insights for future DFT development.

Findings

SIE increases with the number of nuclei in a linear arrangement.

Molecular shape significantly impacts the magnitude of SIE.

Higher orbital occupation leads to increased SIE, especially in excited states.

Abstract

Density Functional Theory (DFT) sees prominent use in computational chemistry and physics, however, problems due to the self-interaction error (SIE) pose additional challenges to obtaining qualitatively correct results. An unphysical energy an electron exerts on itself, the SIE impacts most practical DFT calculations. We conduct an in-depth analysis of the one-electron SIE in which we replicate delocalization effects for simple geometries. We present a simple visualization of such effects, which may help in future qualitative analysis of the one-electron SIE. By increasing the number of nuclei in a linear arrangement, the SIE increases dramatically. We also show how molecular shape impacts the SIE. Two and three dimensional shapes show an even greater SIE stemming mainly from the exchange functional with some error compensation from the one-electron error, which we previously defined [Phys. Chem. Chem. Phys. 22, 15805 (2020)]. Most tested geometries are affected by the functional error, while some suffer from the density error. For the latter we establish a potential connection with electrons being unequally delocalized by the DFT methods. We also show how the the SIE increases if electrons occupy higher-lying atomic orbitals; seemingly one-electron SIE free methods in a ground are no longer SIE free in excited states, which is an important insight for some popular, non-empirical DFAs. We conclude that the erratic behavior of the SIE in even the simplest geometries shows that robust density functional approximations are needed. Our test systems can be used as a future benchmark or contribute towards DFT development.