code: def dr( self, cell_lengths ):
          
          half_cell_lengths = cell_lengths / 2.0
          this_dr = self.final_site.r - self.initial_site.r
          for i in range( 3 ):
              if this_dr[ i ] > half_cell_lengths[ i ]:
                  this_dr[ i ] -= cell_lengths[ i ]
              if this_dr[ i ] < -half_cell_lengths[ i ]:
                  this_dr[ i ] += cell_lengths[ i ]
          return this_dr
docs: Particle displacement vector for this jump
  
          Args:
              cell_lengths (np.array(x,y,z)): Cell lengths for the orthogonal simulation cell.
  
          Returns
              (np.array(x,y,z)): dr