In the case where you have a default link which is made up of a sum of several potentials, i.e.:

[ link ]
resname $protein_resnames
[ dihedrals ]
-BB BB +BB ++BB 9 0 10 1
-BB BB +BB ++BB 9 20 10 1 {"version": 1}

And you want to replace it in a specific case (particular aminoacid for example) by a link made up of a single potential. i.e.:

[ link ]
resname $protein_resnames
[ dihedrals ]
-BB BB +BB ++BB 1 90 70 1

You are forced to introduce a second potential with fc=0 in order to override the second part of the default link. i.e.:

[ link ]
resname $protein_resnames
[ dihedrals ]
-BB BB +BB ++BB 9 90 70 1
-BB BB +BB ++BB 9  0  0 1

Otherwise, you end up with a mix of the default and the override. Would be cool (and help clean up .ff files) if we could somehow replace the entire link instead. Currently, we are ending up with dirtier .ff and .itp files full of "zeroed" potentials.