= Split-site disorder model
A \b[split-site disorder model] represents alternative atomic positions explicitly within an average <crystal structure>. Each alternative has its own coordinate and <crystallographic site occupancy>. For one atom distributed between exactly two alternatives, with no vacancy, the occupancies sum to1. Correlated alternative arrangements can be collected into disorder groups: atoms within one group occur together, while different groups in the same assembly represent alternatives.<Atom-site symmetry and disorder in the IUCr core CIF dictionary>[`_atom_site_disorder_group`, Definition]{ref}
For an original scattering example, put the alternatives at fractional coordinates $(0,0,0)$ and $(1/2,0,0)$ with occupancies0.7 and0.3. Give both the same real atomic scattering amplitude $f$, and omit all other atoms and displacement attenuation. The coherent average contribution to a reflection $(h00)$ is
$$
F_h=f\bigl(0.7+0.3e^{\pi ih}\bigr).
$$
For an <even integer> $h$, this equals $f$; for an <odd integer> $h$, it equals $0.4f$. The corresponding squared amplitudes are $f^2$ and $0.16f^2$. Treating both alternatives as fully occupied would predict a different <structure factor>. This calculation concerns the periodic average contribution, not a complete description of diffuse scattering from local fluctuations.
A <split-site disorder model> is useful when an <electron density map> supports distinct alternative locations instead of one broad peak. An <anisotropic atomic displacement parameter> can describe directional broadening, but does not by itself encode two mutually exclusive chemical arrangements.
Occupancy constraints depend on the physical model: alternatives for a fully present atom sum to1, whereas a model admitting vacancies need not. An average <crystal structure> alone also cannot establish whether the alternatives exchange during the measurement, occur in different cells, or have particular local correlations.
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