code: def center_atoms(self):
          
          nodes = set()
          for n, atom in self.atoms():
              if atom._reactant != atom._product:
                  nodes.add(n)
  
          for n, m, bond in self.bonds():
              if bond._reactant != bond._product:
                  nodes.add(n)
                  nodes.add(m)
  
          return list(nodes)
docs: get list of atoms of reaction center (atoms with dynamic: bonds, charges, radicals).