def write_thermal_expansion(self, filename="thermal_expansion.dat"):
        """Write thermal expansion vs temperature in file."""
        w = open(filename, "w")
        for i in range(self._len):
            w.write(
                "%25.15f %25.15f\n"
                % (self._temperatures[i], self._thermal_expansions[i])
            )
        w.close()