code: def calculate_centre_of_mass(self):
          
          self.centre_of_mass = center_of_mass(self.elements, self.coordinates)
          self.properties['centre_of_mass'] = self.centre_of_mass
          return self.centre_of_mass
docs: Return the xyz coordinates of the centre of mass of a molecule.
  
          Returns
          -------
          :class:`numpy.array`
              The centre of mass of the molecule.