code: def dr( self, atom1, atom2 ):
          
          return self.cell.dr( atom1.r, atom2.r )
docs: Calculate the distance between two atoms.
  
          Args:
              atom1 (vasppy.Atom): Atom 1.
              atom2 (vasppy.Atom): Atom 2.
  
          Returns:
              (float): The distance between Atom 1 and Atom 2.