The chiral volume, Vc, for chiral centres that involve a chiral
               atom bonded to three non-hydrogen atoms and one hydrogen atom.
               Vc = V1 * (V2 X V3)
               V1 = the vector distance from the atom specified by
                    _chem_comp_chir.atom_id to the first atom in the
                    CHEM_COMP_CHIR_ATOM list
               V2 = the vector distance from the atom specified by
                    _chem_comp_chir.atom_id to the second atom in the
                    CHEM_COMP_CHIR_ATOM list
               V3 = the vector distance from the atom specified by
                    _chem_comp_chir.atom_id to the third atom in the
                    CHEM_COMP_CHIR_ATOM list
               *  = the vector dot product
               X  = the vector cross product