Estimated H-atom anisotropic displacement parameters: a comparison between different methods and with neutron...

Area of Science:

• Crystallography
• Materials Science
• Computational Chemistry

Background:

• Accurate estimation of anisotropic displacement parameters (ADPs) for hydrogen atoms is crucial for detailed crystallographic analyses, particularly in charge-density studies.
• Existing methods for deriving hydrogen ADPs have limitations and varying degrees of applicability.

Purpose of the Study:

• To compare three recent procedures for estimating hydrogen atom ADPs.
• To introduce a revised and extended library (SHADE2) for hydrogen atom mean-square displacements.
• To assess the utility of the SHADE2 library in charge-density studies.

Main Methods:

• Comparison of hydrogen ADPs derived from three novel procedures against neutron diffraction data.
• Development and application of the SHADE2 library, an updated resource for internal H-atom mean-square displacements.
• Evaluation of rigid-body models for heavy atoms in conjunction with the SHADE2 library.

Main Results:

• All tested methods demonstrate capability in yielding excellent hydrogen ADP estimates for benchmark systems.
• Systematic discrepancies were identified for specific atom types across the methods.
• The SHADE2 library, integrated into the SHADE web server, shows substantial improvement over original SHADE results.
• A segmented rigid-body description of heavy atoms offers only marginal improvement when using SHADE2.

Conclusions:

• The SHADE2 library, now part of the SHADE web server, is recommended for routine estimation of hydrogen ADPs in molecular crystal charge-density studies.
• Widespread adoption of SHADE2 is expected to highlight remaining deficiencies and help mitigate inherent biases in current studies.
• The revised library enhances the applicability and accuracy of hydrogen ADP estimation in crystallographic research.