Vibrational Helmholtz free energy, A_vib(V, T).
        Eq(4) in doi.org/10.1016/j.comphy.2003.12.001

        Args:
            temperature (float): temperature in K
            volume (float)

        Returns:
            float: vibrational free energy in eV

def vibrational_free_energy(self, temperature, volume):
        """
        Vibrational Helmholtz free energy, A_vib(V, T).
        Eq(4) in doi.org/10.1016/j.comphy.2003.12.001

        Args:
            temperature (float): temperature in K
            volume (float)

        Returns:
            float: vibrational free energy in eV
        """
        y = self.debye_temperature(volume) / temperature
        return self.kb * self.natoms * temperature * (
            9./8. * y + 3 * np.log(1 - np.exp(-y)) - self.debye_integral(y))