How do density functionals affect the Hirshfeld atom refinement?

Area of Science:

• Computational chemistry
• Quantum crystallography
• Materials science

Background:

• Hybrid density functionals are widely used in computational chemistry.
• Hirshfeld atom refinement (HAR) is a method for determining molecular structures.
• The influence of Hartree-Fock (HF) exchange on HAR is not well understood.

Purpose of the Study:

• To investigate the effect of varying Hartree-Fock (HF) exchange in hybrid density functionals on the Hirshfeld atom refinement (HAR) of urea and oxalic acid dihydrate.
• To explore the impact of different basis sets, computational methods (MP2, HF), and cluster sizes on HAR results.
• To compare HAR outcomes with neutron diffraction data.

Main Methods:

• Application of hybrid density functionals with varying HF exchange percentages.
• Hirshfeld atom refinement (HAR) on urea and oxalic acid dihydrate.
• Utilizing different basis sets, MP2 and HF methods, and cluster sizes for bulk effect modeling.

Main Results:

• The amount of HF exchange primarily affects the refinement parameters of hydrogen atoms.
• Unlike geometry optimizations, optimal HF exchange mixtures for HAR do not necessarily yield the best agreement with neutron diffraction.
• Non-hydrogen bond lengths are insensitive to method or basis set, while X-H bond lengths increase with HF exchange.
• Thermal ellipsoids tend to shrink with increasing HF exchange, particularly for hydrogen atoms in strong hydrogen bonds.

Conclusions:

• The optimal computational parameters for quantum crystallography via HAR may differ from those used in standard quantum chemistry calculations.
• Development of density functionals and basis sets for quantum crystallography might require a distinct approach.
• Findings suggest a divergence in computational strategies for accurate structural refinement versus electronic property prediction.