code: def _approximate_unkown_bond_lengths(self):
          
          dataset = self.lengths[BOND_SINGLE]
          for n1 in periodic.iter_numbers():
              for n2 in periodic.iter_numbers():
                  if n1 <= n2:
                      pair = frozenset([n1, n2])
                      atom1 = periodic[n1]
                      atom2 = periodic[n2]
                      #if (pair not in dataset) and hasattr(atom1, "covalent_radius") and hasattr(atom2, "covalent_radius"):
                      if (pair not in dataset) and (atom1.covalent_radius is not None) and (atom2.covalent_radius is not None):
                          dataset[pair] = (atom1.covalent_radius + atom2.covalent_radius)
docs: Completes the bond length database with approximations based on VDW radii