Density functional theory with fractional orbital occupations.

Area of Science:

• Quantum Chemistry
• Computational Materials Science
• Electronic Structure Theory

Background:

• Conventional Kohn-Sham (KS) density functional theory (DFT) struggles with systems exhibiting strong static correlation.
• Accurate modeling of ground states in many-electron systems with strong static correlation remains a significant challenge.

Purpose of the Study:

• To develop and validate a novel DFT approach incorporating fractional orbital occupations for improved description of strong static correlation.
• To assess the performance of this new DFT method against established methods for both single- and multi-reference systems.
• To apply the computationally efficient DFT method to investigate challenging electronic properties of acenes.

Main Methods:

• Development of a density functional theory (DFT) framework that explicitly includes fractional orbital occupations.
• Application of the local density approximation (LDA) within this new DFT framework (DFT-LDA).
• Validation against established methods (KS-LDA) and experimental/high-level ab initio data for various molecular systems, including H2, N2, ethylene, and acenes.

Main Results:

• The proposed DFT-LDA method demonstrates improved accuracy over KS-LDA for multi-reference systems (e.g., H2, N2 dissociation, twisted ethylene).
• Performance is comparable to KS-LDA for single-reference systems (e.g., reaction energies, equilibrium geometries).
• Calculated singlet-triplet (ST) energy gaps for acenes show good agreement with experimental and high-level ab initio data, revealing a monotonic decrease with increasing chain length.

Conclusions:

• The DFT with fractional orbital occupations provides a computationally efficient and accurate approach for studying systems with strong static correlation.
• This method accurately predicts singlet-triplet energy gaps in acenes, indicating they approach zero for large polyacenes.
• Ground states of large acenes are predicted to be polyradical singlets based on calculated active orbital occupations.