code: def calculate_pore_volume_opt(self, **kwargs):
          
          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
docs: 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.