text: λ2 
  6Jcluster h 
  λ2 
  6Jcluster h  9 + 9r2 
  16 
  9 + 9r2 
  16 
  
  + 2r 
  
  Pjk(Sj1 
  j τ x 
  
  Sj2)(Sk1  Sk2) 
  ·  · 
  
  + (r/2)τ x  r/2 
  k − 
  
  r 
  Pjk(Sj1 
  Sj2 + Sk1  Sk2) 
  Pjki.  −  ·  · 
  
  (r λ2)/(12Jcluster) = 
  Sj2 + Sk1 
  So we can choose 
  the last intra-cluster Sj1 
  order perturbation. 
  Jx, and include 
  Sk2 term in the ﬁrst 
  −  − 
  ·  · 