              The [2] element of the 3 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|