code: def _get_rho(self, v):
          
          v_mol = vol_uc2mol(v, self.z)  # in m^3
          rho = self.mass / v_mol * 1.e-6  # in g/cm^3
          return rho
docs: convert unit-cell volume in A^3 to density in g/cm^3
  
          :param v: unit cell volume in A^3
          :return: density in g/cm^3
  
          :note: internal function