code: def add_bond(self, particle_pair):
          
          if self.root.bond_graph is None:
              self.root.bond_graph = BondGraph()
  
          self.root.bond_graph.add_edge(particle_pair[0], particle_pair[1])
docs: Add a bond between two Particles.
  
          Parameters
          ----------
          particle_pair : indexable object, length=2, dtype=mb.Compound
              The pair of Particles to add a bond between