Data items in the GEOM_TORSION category record details about
torsion angles as calculated from the
contents of the ATOM, CELL and SYMMETRY data.
The vector direction _geom_torsion.atom_site_id_2 to
_geom_torsion.atom_site_id_3 is the viewing direction, and the
torsion angle is the angle of twist required to superimpose the
projection of the vector between site 2 and site 1 onto the
projection of the vector between site 3 and site 4. Clockwise
torsions are positive, anticlockwise torsions are negative.
Ref: Klyne, W. & Prelog, V. (1960). Experientia, 16, 521-523.