              Data items in the GEOM_TORSION category record details about\
               molecular and crystal 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 site2-site1 onto the projection of the\
               vector site3-site4. Clockwise torsions are positive,\
               anticlockwise torsions are negative.\
\
               Ref: Klyne, W. \& Prelog, V. (1960). Experientia, 16, 521-523.