.. _subs_buildtasks_psfgen_mods_links: links ----- A ``links`` directive is used to specify the addition of a covalent bond between two atoms. The syntax for specifying a link is as follows: ``__-__`` ```` and ```` are the chain IDs, ```` and ```` are the residue numbers of the two residues that form the link, and ```` and ```` are the atom names of the two atoms that form the link. For example, ``A-12_O-C_15_N`` would create a covalent bond between the oxygen atom of residue 12 in chain A and the nitrogen atom of residue 15 in chain C. You can use either 1-byte or 3-byte residue names. Links are not normally needed in a ``mods`` directive of a ``psfgen`` task. However, if you are using a non-published pdb file that does not include metadata indicating non-protein bonds (e.g., a covalent bond between a glycan monomer and a protein), you can use the ``links`` directive to specify each required bond. The ``links`` directive is not needed for standard protein structures.