    # Change residue types of potentially charged residues in
    # selected molecules to properly reflect charge.

    # store current selection
    DefPropRes 'prev_sel' 'selected'
    DefPropMol 'prev_sel' 'selected'

    # change ASP that do not have an HD2 atom to ASP-
    SelectRes 'mol.selected & name = "ASP" & ! exists atom(name = "HD2")'
    ChangeRes atoms 'ASP-'

    # change GLU that do not have an HE2 atom to GLU-
    SelectRes 'mol.selected & name = "GLU" & ! exists atom(name = "HE2")'
    ChangeRes atoms 'GLU-'

    # change HIS+ that do not have an HE2 atom to HIS
    SelectRes 'mol.selected & name = "HIS+" & ! exists atom(name = "HE2")'
    ChangeRes atoms 'HIS'

    # restore previous selection
    SelectRes 'prev_sel'
    SelectMol 'prev_sel'