code: def _get_v(self, rho):
          
          v_mol = self.mass / rho  # in cm^3
          v = vol_mol2uc(v_mol * 1.e-6, self.z)
          print(v)
          return v
docs: convert density in g/cm^3 to unit-cell volume
  
          :param rho: density in g/cm^3
          :return: unit-cell volume in A^3
  
          :note: internal function