= Solution
<Patterson maps> give interatomic displacement vectors rather than absolute positions. The three strong half-cell vectors are consistent with a face-centered four-point Se substructure. One permissible coordinate set is
$$
(0,0,0),\quad(0,\tfrac12,\tfrac12),\quad
(\tfrac12,0,\tfrac12),\quad(\tfrac12,\tfrac12,0).
$$
Each nonzero vector occurs four times among the twelve ordered Se-Se pairs, explaining equal weights. These positions form a valid four-site orbit of the specified <space group> in a suitable origin convention.
A common translation of all four sites is invisible to a <Patterson map>; absolute coordinates are therefore not uniquely fixed by these three peaks. Subsequent phases are stated in the origin convention above. The remaining atoms need not have face-centered translational symmetry merely because the dominant Se substructure does.
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