Secondary kinetic peak in the Kohn-Sham potential and its connection to the response step

TL;DR

This paper investigates specific features of the Kohn-Sham potential in a 1D model of a stretched heteronuclear molecule, revealing new peaks and steps linked to the response behavior and providing analytical explanations for these phenomena.

Contribution

It uncovers and explains a secondary peak in the kinetic potential and its relation to the response step, extending understanding of Kohn-Sham potential features in molecular dissociation.

Findings

Identification of a secondary peak in the kinetic potential.

Explanation of the peaks' role in delineating the response region.

Validation of the Heitler-London approximation at large internuclear distances.

Abstract

We consider a prototypical 1D model Hamiltonian for a stretched heteronuclear molecule and construct individual components of the corresponding KS potential, namely: the kinetic, the N - 1, and the conditional potentials. These components show very special features, such as peaks and steps, in regions where the density is drastically low. Some of these features are quite well known, whereas others, such as a secondary peak in the kinetic potential or a second bump in the conditional potential, are less or not known at all. We discuss these features building on the analytical model treated in J. Chem. Theory Comput. 14, 4151 (2018). In particular, we provide an explanation for the underlying mechanism which determines the appearance of both peaks in the kinetic potential and elucidate why these peaks delineate the region over which the plateau structure, due to the N - 1 potential, stretches. We assess the validity of the Heitler-London Ansatz at large but finite internuclear distance, showing that, if optimal orbitals are used, this model is an excellent approximation to the exact wavefunction. Notably, we find that the second natural orbital presents an extra node very far out on the side of the more electronegative atom.