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The frequent practice of combining X-ray and neutron diffraction for distinguishing vibrational smearing from charge deformation due to chemical bonding is neither necessary nor completely satisfactory. The two effects occur principally in distinct regions of real and reciprocal space. They can be distinguished by X-ray data alone provided these extend to reciprocal radii d* > 2 Å- 1 and provided the refinement program allows explicitly for the bonding deformation. The success of this separation can be tested by comparison of the vibration ellipsoids of bonded atom pairs, which should have equal amplitudes in the bond direction. Application of this 'rigid-bond' test to four recent refinements using the charge deformation model shows the vibration parameters to be largely unbiased, as those from parallel spherical-atom refinements are not. Hydrogen vibration parameters cannot be derived from X-ray data because of large deformation densities at the nuclei; nor do they satisfy the rigid-bond postulate.
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