[Chimera-users] Definition of residues involved in binding ligand

Dear reader, I have a pdb file of a protein with bound ADP and the file contains nothing but atomic coordinates and a few CONECT lines. If I open the file, the residues involved in binding ADP are shown, but I would like to know how Chimera knows this. I assume it is some VdW vicinity/overlap perhaps, or all residues within a certain distance from the ADP molecule, but I would like to know exactly how it is defined. I have tried to look for it in the documentation but haven't been successful so far. Does someone know the answer or a link to where it has been discussed previously? Many thanks in advance! Maarten