Data items in the CHEMICAL_CONN_ATOM category would not, in
               general, be used in a macromolecular CIF. See instead the
               ENTITY data items.

               Data items in the CHEMICAL_CONN_ATOM and CHEMICAL_CONN_BOND
               categories record details about the 2D chemical structure of the
               molecular species. They allow a 2D chemical diagram to be
               reconstructed for use in a publication or in a database search
               for structural and substructural relationships.

               The CHEMICAL_CONN_ATOM data items provide information about the
               chemical properties of the atoms in the structure. In cases
               where crystallographic and molecular symmetry elements coincide
               they must also contain symmetry-generated atoms, so that the
               CHEMICAL_CONN_ATOM and CHEMICAL_CONN_BOND data items will always
               describe a complete chemical entity.