Density-Dependent Onset of the Long-Range Exchange: A Key to Donor-Acceptor Properties

TL;DR

This paper introduces a density-dependent approach to improve the accuracy of density functional theory in predicting orbital energies and charge-transfer properties in organic donor-acceptor systems by controlling the onset of long-range exchange.

Contribution

A new paradigm for range-separated functionals that uses electron-exchange hole separation to better predict one-particle states without explicit ionization.

Findings

Accurately predicts orbital gaps and charge-transfer excitations.

Provides physically sound descriptions of electron density tails.

Improves alignment of orbital energies with ionization potentials.

Abstract

Quantum mechanical methods based on the density functional theory (DFT) offer a realistic possibility of first-principles design of organic donor-acceptor systems and engineered band-gap materials. This promise is contingent upon the ability of DFT to predict one-particle states accurately. Unfortunately, approximate functionals fail to align the orbital energies with ionization potentials. We describe a new paradigm for achieving this alignment. In the proposed model, an average electron-exchange hole separation controls the onset of the orbital-dependent exchange in approximate range-separated functionals. The correct description of one-particle states is thus achieved without explicit electron removal or attachment. Extensive numerical tests show that the proposed method provides physically sound orbital gaps and leads to excellent predictions of charge-transfer excitations and other properties critically depending on the tail of the electron density.