text: form 
  
  J ′(QE 
  1 f E  1 + QE  2 f E 
  2 ) 
  
  where J ′ is the derivative of Heisenberg coupling Jcluster 
  between two spins ℓ and m with respect to their distance 
  rℓm, J ′ = dJcluster/drℓm; QE 
  1,2 are the generalized coor- 
  dinates of these two modes; and the functions f E 
  1,2 are 