IUCr journals

Anisotropic displacement parameters for H atoms using an ONIOM approach

Acta Cryst. (2006). B62, 875–888 [doi.org/10.1107/S0108768106020787]

[ORTEP]ORTEP comparison between TLS+ONIOM and neutron ADPs for 1-methyluracil at 21 K (99% surfaces). Numbers are values of a similarity index that measures overlap between reference and model probability density functions.
Charge-density analyses of molecular crystals require an accurate description of the motion of all nuclei, including hydrogens, yet nearly 80% of recent studies treat motion of hydrogen atoms isotropically. Using widely available software we combine ab initio/molecular mechanics ONIOM cluster calculations with rigid body fits to X-ray ADPs for heavy atoms, to derive ADPs for hydrogens for a variety of molecular crystals. The agreement with neutron diffraction results is impressive, and the method offers considerable promise in future charge-density studies of molecular crystals.

A.E. Whitten and M.A. Spackman