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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def plot_helmholtz_free_energy(self, tmin, tmax, ntemp, ylim=None, **kwargs):
 """ Plots the vibrational contribution to the Helmoltz free energy in a temperature range. Args: tmin: minimum temperature tmax: maximum temperature ntemp: number of steps ylim: tuple specifying the y-axis limits. kwargs: kwargs passed to the matplotlib function 'plot'. Returns: matplotlib figure """

        temperatures = np.linspace(tmin, tmax, ntemp)

        if self.structure:
            ylabel = r"$\Delta F$ (kJ/mol)"
        else:
            ylabel = r"$\Delta F$ (kJ/mol-c)"

        fig = self._plot_thermo(self.dos.helmholtz_free_energy, temperatures, ylabel=ylabel, ylim=ylim,
                                factor=1e-3, **kwargs)

        return fig