def write_helmholtz_volume(self, filename="helmholtz-volume.dat"):
        """Write Helmholtz free energy vs volume in file."""
        w = open(filename, "w")
        for i, (t, ep, fe) in enumerate(
            zip(self._temperatures, self._equiv_parameters, self._free_energies)
        ):
            if i == self._len:
                break

            w.write("# Temperature: %f\n" % t)
            w.write("# Parameters: %f %f %f %f\n" % tuple(ep))
            for j, v in enumerate(self._volumes):
                w.write("%20.15f %25.15f\n" % (v, fe[j]))
            w.write("\n\n")
        w.close()