text: λx QE 
  QE 
  1k 
  HSL = 
  1j · 
  X 
  cluster 
  
  + 
  X 
  y−links <jk> 
  
  λz QE 
  1jQE  1kQE 
  2k 
  + 
  2j · 
  X 
  z−links <jk> 
  
  1j is the generalized coordinate for the QE 
  2j, QE  1k, QE 
  
  − 
  
  Hamiltonian HSL explicitly written as, 