Electron localisation descriptors in ONETEP: a tool for interpreting localisation and bonding in large-scale DFT calculations

Abstract

Electron localisation descriptors, such as the electron localisation function (ELF) and localised orbital locator (LOL) provide a visual tool for interpreting the results of electronic structure calculations. The descriptors produce a quantum valence shell electron pair repulsion (VSEPR) representation, indicating the localisation of electron pairs into bonding pairs and lone pairs in single molecules, coordination compounds and crystalline solids. We have implemented the ELF and LOL within ONETEP, a DFT code designed to perform calculations on systems containing thousands of atoms with plane-wave accuracy. This is possible using a linear-scaling formulation of DFT in which the Kohn–Sham orbitals are expressed in terms of a set of strictly localised non-orthogonal generalised Wannier functions (NGWFs), themselves expanded in a psinc basis set. In this paper, we describe our implementation and explore the chemical insights offered by electron localisation descriptors in ONETEP in a range of bonding and nonbonded situations.