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

def _get_rho(self, v):
        """
        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
        """
        v_mol = vol_uc2mol(v, self.z)  # in m^3
        rho = self.mass / v_mol * 1.e-6  # in g/cm^3
        return rho