def heat_capacity_P_polyfit(self):
        """Return heat capacities at constant pressure at temperatures.

        Volumes are computed in another way to heat_capacity_P_numerical
        for the better numerical behaviour. But this does not work
        when temperature dependent electronic_energies is supplied.

        """
        if self._electronic_energies.ndim == 1:
            return self._cp_polyfit[: self._len]
        else:
            return None