Density-Corrected Density Functional Theory for Open Shells: How to Deal with Spin Contamination.

Area of Science:

• Quantum chemistry
• Computational materials science
• Theoretical chemistry

Background:

• Density functional theory (DFT) is a standard method for predicting chemical properties.
• Using Hartree-Fock (HF) densities can improve DFT energetics in specific scenarios.
• Density-corrected (DC) DFT formalizes this approach.
• Unrestricted Hartree-Fock (UHF) densities can lead to poor results when spin contamination is significant.

Purpose of the Study:

• To compare the performance of restricted open-shell Hartree-Fock (ROHF) and unrestricted Hartree-Fock (UHF) densities within DC-DFT.
• To evaluate the impact of spin contamination on the accuracy of different DFT functionals and DC-DFT methods.
• To refine the DC-DFT algorithm for cases with severe spin contamination.

Main Methods:

• Comparison of ROHF and UHF densities across 13 DFT functionals.
• Application of two DC-DFT methods.
• Analysis of energy improvements and spin contamination effects.

Main Results:

• ROHF densities significantly outperform UHF densities in cases of strong spin contamination, improving energetics by up to a factor of 3.
• ROHF-based DC-DFT generally shows improvement over self-consistent DFT for most functionals tested.
• The DC(HF)-DFT algorithm was refined to incorporate ROHF densities for improved accuracy in spin-contaminated systems.

Conclusions:

• ROHF densities offer a more robust approach than UHF densities for DC-DFT, particularly in the presence of spin contamination.
• The refined DC-DFT method using ROHF densities provides more accurate energetic predictions for challenging chemical systems.
• This work advances the reliability of DFT calculations for systems prone to spin contamination.