The [3] element of the three-element vector used to transform
               Cartesian coordinates in the ATOM_SITE category to fractional
               coordinates in the same category. The axial alignments of this
               transformation are described in _atom_sites.Cartn_transform_axes.
               The 3x3 rotation is defined in
               _atom_sites.fract_transf_matrix[][].
               |x'|               |11 12 13| |x|              |1|
               |y'|fractional = |21 22 23| |y|Cartesian + |2|
               |z'|               |31 32 33| |z|              |3|