##
Excluded pairs

You can also exclude certain pairs of atoms from the nonbonded list. These Python objects are added to the Restraints.excluded_pairs list.

`excluded_pair(atom_id1, atom_id2)`
Excludes the given two atoms from the nonbonded list.

Example: examples/scoring/excluded_pair.py

      # Demonstrate the use of excluded pairs.

      # In this example we approximate a disulfide linkage by creating a distance
      # restraint between two SG atoms in CYS residues. Since these atoms are in
      # different residues, ordinarily Modeller will calculate a van der Waals
      # (soft sphere) interaction between them. We use an excluded pair to prevent
      # this interaction from being calculated, as otherwise it will conflict
      # with the new distance restraint.

      # Note that this is an example only; ordinarily a DISU patch would be used
      # to create a disulfide linkage. The DISU patch has the advantage that it
      # restrains the angles and dihedrals involved with the SG-SG bond, and also
      # excludes these atom pairs from van der Waals interaction.

      from modeller import *
      from modeller.scripts import complete_pdb
      from modeller.optimizers import conjugate_gradients

      env = environ()
      env.io.atom_files_directory = ['../atom_files']
      env.edat.dynamic_sphere = True

      env.libs.topology.read(file='$(LIB)/top_heav.lib')
      env.libs.parameters.read(file='$(LIB)/par.lib')

      code = '1fas'
      mdl = complete_pdb(env, code)

      atom1 = mdl.atoms['SG:3:A']
      atom2 = mdl.atoms['SG:22:A']
      mdl.restraints.add(forms.gaussian(group=physical.xy_distance,
                                        mean=2.0, stdev=0.1,
                                        feature=features.distance(atom1, atom2)))
      mdl.restraints.excluded_pairs.append(excluded_pair(atom1, atom2))

      # Retain stereochemistry
      atmsel = selection(mdl)
      mdl.restraints.make(atmsel, restraint_type='stereo', spline_on_site=False)

      # Optimize the model with CG
      cg = conjugate_gradients(output='REPORT')
      cg.optimize(atmsel, max_iterations=100)

      mdl.write(file=code+'.expair.pdb')