Return the intrinsic pore volume (for the optimised pore centre).

        Similarly to :func:`calculate_pore_volume` this method returns the
        the volume intrinsic pore diameter, however, for the
        :func:`calculate_pore_diameter_opt` returned value.

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

def calculate_pore_volume_opt(self, **kwargs):
        """
        Return the intrinsic pore volume (for the optimised pore centre).

        Similarly to :func:`calculate_pore_volume` this method returns the
        the volume intrinsic pore diameter, however, for the
        :func:`calculate_pore_diameter_opt` returned value.

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

        """
        self.pore_volume_opt = sphere_volume(
            self.calculate_pore_diameter_opt(**kwargs) / 2)
        self.properties['pore_volume_opt'] = self.pore_volume_opt
        return self.pore_volume_opt