A value that should be taken as a potential target value for the
torsion angle associated with the specified atoms, expressed as
the distance between the atoms specified by
_chem_link_tor.atom_id_1 and _chem_link_tor.atom_id_4 in the
referenced record in the CHEM_LINK_TOR list. Note that the
torsion angle cannot be fully specified by a distance (for
instance, a torsion angle of -60 degree will yield the same
distance as a 60 degree angle). However, the distance
specification can be useful for refinement in situations in
which the angle is already close to the desired value.