The [1] 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|