Fragment occupations in partition Density Functional Theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Partition Density Functional Theory (PDFT) allows for the calculation of molecular properties using Kohn-Sham calculations on isolated molecular fragments.
• PDFT determines unique fragment densities by minimizing the sum of fragment energies under the constraint that fragment densities reconstruct the total molecular density.

Purpose of the Study:

• To investigate the relationship between fragment densities derived from different fragmentation schemes in PDFT.
• To analyze the nature of fragment occupations in PDFT, particularly concerning their dependence on inter-fragment separation and chemical potential equalization.

Main Methods:

• Utilizing Kohn-Sham calculations on isolated molecular fragments.
• Applying the PDFT framework to find fragment densities by minimizing energy sums with density constraints.
• Examining fragment occupations resulting from various fragmentation strategies and inter-fragment distances.

Main Results:

• A "near-additivity" property was observed for fragment densities across different fragmentation choices.
• Fragment occupations approach integer values as inter-fragment separation increases.
• Strictly integer occupations emerge at small inter-fragment separations.
• Cases where fragment chemical potentials cannot be equalized result in locked integer occupations.

Conclusions:

• PDFT fragment densities exhibit near-additivity, indicating robustness to fragmentation choices.
• Fragment occupations in PDFT are sensitive to inter-fragment separation and chemical potential equalization, transitioning to integer values under specific conditions.
• The findings provide insights into the behavior of electron density and occupations in fragmented molecular systems within PDFT.