code: def minimum_image_dr( self, r1, r2, cutoff=None ):
          
          delta_r_vector = self.minimum_image( r1, r2 )
          return( self.dr( np.zeros( 3 ), delta_r_vector, cutoff ) )
docs: Calculate the shortest distance between two points in the cell, 
          accounting for periodic boundary conditions.
  
          Args:
              r1 (np.array): fractional coordinates of point r1.
              r2 (np.array): fractional coordinates of point r2.
              cutoff (:obj: `float`, optional): if set, return zero if the minimum distance is greater than `cutoff`. Defaults to None.
  
          Returns:
              (float): The distance between r1 and r2.