Phase contributions to the deformation density

The true phases of the structure factors will, in general, be different from the phases calculated with the independent-atom model. In centrosymmetric structures, with phases restricted to 0 or n, only very few weak reflections are affected. In acentric structures, only the reflections of centrosymmetric projections, such as the hkO, hOl, and Okl reflections in the space group P212,21, are invariant. 

The effect of phase differences on dynamic deformation density may be estimated as follows (Souhassou et al. 1991). The amplitudes of the deformation density Fourier series may be written as (Fig. 5.9) 

The deformation density Fourier series can thus be written as 

In other words, the deformation density can be separated into structure-factor magnitude and structure-factor phase contributions  

With Acp = (pltM — pref, the phase-factor difference in the phase contribution can be rewritten as 

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