Strong Correlation and Charge Localization in Kohn-Sham Theories with Fractional Orbital Occupations.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Materials Science

Background:

• Static correlation and delocalization errors are significant challenges in electronic structure calculations.
• Accurate prediction of molecular properties like dipole moments is crucial for various applications.

Purpose of the Study:

• To investigate static correlation and delocalization errors in electronic structure methods.
• To compare state-of-the-art approaches with a generalized valence-bond ansatz using an optimized effective potential (OEP).

Main Methods:

• Utilized a generalized valence-bond ansatz with orbitals and fractional occupations as variational parameters.
• Employed an optimized effective potential (OEP) to derive the exchange-correlation functional.
• Compared OEP results with existing density functional theory (DFT) and reduced density matrix functional theory (RDMFT) approximations.

Main Results:

• Methods with good energies can still exhibit substantial delocalization errors affecting molecular density and dipole moments.
• The OEP shows step and peak features, crucial for preventing charge delocalization, similar to exact Kohn-Sham potentials.
• Common RDMFT approximations lack these features, leading to delocalization errors comparable to DFT approximations.

Conclusions:

• Delocalization errors arise from artificial mixing of ground and charge-transfer excited states.
• Discontinuities in occupation numbers are essential to avoid these errors.
• The OEP provides a more accurate description by preventing charge delocalization.