A new approach to local hardness

TL;DR

This paper introduces a novel method for defining local hardness in molecules, addressing previous ambiguities and inconsistencies, and demonstrating promising numerical results that improve the correlation with local softness indicators.

Contribution

The paper proposes a new approach to define and evaluate local hardness using a local chemical potential, ensuring it integrates to the global hardness and correlates inversely with the Fukui function.

Findings

The new local hardness integrates to the global hardness.

It shows an inverse relation with the Fukui function.

Numerical tests indicate promising results.

Abstract

The applicability of the local hardness as defined by the derivative of the chemical potential with respect to the electron density is undermined by an essential ambiguity arising from this definition. Further, the local quantity defined in this way does not integrate to the (global) hardness - in contrast with the local softness, which integrates to the softness. It has also been shown recently that with the conventional formulae, the largest values of local hardness do not necessarily correspond to the hardest regions of a molecule. Here, in an attempt to fix these drawbacks, we propose a new approach to define and evaluate the local hardness. We define a local chemical potential, utilizing the fact that the chemical potential emerges as the additive constant term in the number-conserving functional derivative of the energy density functional. Then, differentiation of this local chemical potential with respect to the number of electrons leads to a local hardness that integrates to the hardness, and possesses a favourable property; namely, within any given electron system, it is in a local inverse relation with the Fukui function, which is known to be a proper indicator of local softness in the case of soft systems. Numerical tests for a few selected molecules and a detailed analysis, comparing the new definition of local hardness with the previous ones, show promising results.