Return the intrinsic pore volume.

        Returns
        -------
        :class:`float`
            The intrinsic pore volume.

def calculate_pore_volume(self):
        """
        Return the intrinsic pore volume.

        Returns
        -------
        :class:`float`
            The intrinsic pore volume.

        """
        self.pore_volume = sphere_volume(self.calculate_pore_diameter() / 2)
        self.properties['pore_volume'] = self.pore_volume
        return self.pore_volume