h_add uses a primitive algorithm to add hydrogens onto a molecule.
Usage and Algorithm Notes
PyMOL fills missing valences but does no optimization of hydroxyl rotamers. Also, many crystal structures have bogus or arbitrary ASN/GLN/HIS orientations. Getting the proper amide rotamers and imidazole tautomers & protonation states assigned is a nontrivial computational chemistry project involving electrostatic potential calculations and a combinatorial search. There's also the issue of solvent & counter-ions present in systems like aspartyl proteases with adjacent carboxylates .
Accessing Through GUI
# normal usage h_add (selection) # API usage cmd.h_add( string selection="(all)" )
see also h_fill