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

def _get_v(self, rho):
        """
        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
        """
        v_mol = self.mass / rho  # in cm^3
        v = vol_mol2uc(v_mol * 1.e-6, self.z)
        print(v)
        return v