Approximate anisotropic displacement parameters for H atoms in molecular crystals

Anisotropic displacement parameters for H atoms in molecular crystals are calculated with a simplified model that uses approximate vibrational modes, neglecting correlations between internal and external contributions to the mean square motions. These calculated values agree within a few esds with the ones obtained from neutron diffraction; used in charge density multipolar refinements against X-ray diffraction data, they yield electron densities and derived electrostatic properties that are more accurate than the ones obtainable with the traditional isotropic description. For example, electric field gradients at the H nuclei of L-Alanine are in quantitative agreement with Nuclear Quadrupole Resonance results.