code: def calculate_pore_volume(self):
          
          self.pore_volume = sphere_volume(self.calculate_pore_diameter() / 2)
          self.properties['pore_volume'] = self.pore_volume
          return self.pore_volume
docs: Return the intrinsic pore volume.
  
          Returns
          -------
          :class:`float`
              The intrinsic pore volume.