Ionization potentials and fundamental gaps in atomic systems from the Ensemble-DFT approach.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Materials Science

Background:

• Kohn-Sham density functional theory (DFT) calculations require accurate exchange-correlation (xc) functionals.
• Standard DFT approximations struggle to predict ionization potentials (IP) and fundamental gaps accurately.
• These inaccuracies stem from the lack of piecewise-linearity and the derivative discontinuity in approximate xc functionals.

Purpose of the Study:

• To evaluate the ensemble generalization procedure for improving DFT predictions of IP and fundamental gaps.
• To assess the accuracy of generalized ensemble approximations for atoms and first ions across the Periodic Table.
• To identify trends in accuracy based on the character of frontier orbitals.

Main Methods:

• Applied ensemble generalization to local spin-density and Perdew-Burke-Ernzerhof approximations.
• Studied a wide range of atoms and first ions with s-, p-, and d-orbital character.
• Analyzed the impact of frontier orbital character on the accuracy of the ensemble method.

Main Results:

• Ensemble generalization significantly improves the prediction of IP and fundamental gaps for many systems.
• The method shows high accuracy for systems dominated by s- and p-orbitals.
• Accuracy is notably reduced when d-orbitals are involved in the frontier orbitals.

Conclusions:

• Ensemble generalization is a computationally efficient method to enhance the predictive power of approximate xc functionals.
• The accuracy of the ensemble method is strongly dependent on the nature of the frontier orbitals.
• Further research is needed to address the limitations for systems involving d-orbitals and to explore potential improvements.