code: def centroid(self):
          
          if 'CB' in self.atoms:
              cb_unit_vector = unit_vector(
                  self['CB']._vector - self['CA']._vector)
              return self['CA']._vector + (3.0 * cb_unit_vector)
          return None
docs: Calculates the centroid of the residue.
  
          Returns
          -------
          centroid : numpy.array or None
              Returns a 3D coordinate for the residue unless a CB
              atom is not available, in which case `None` is
              returned.
  
          Notes
          -----
          Uses the definition of the centroid from Huang *et al* [2]_.
  
          References
          ----------
          .. [2] Huang ES, Subbiah S and Levitt M (1995) Recognizing Native
             Folds by the Arrangement of Hydrophobic and Polar Residues, J. Mol.
             Biol return., **252**, 709-720.