text: Hcluster = (Jcluster/2) (S1 + S2 + S3 + S4)2  (2) 
  
  The energy levels should be apparent from this form: 
  one group of spin-2 quintets with energy 3Jcluster, three 
  groups of spin-1 triplets with energy Jcluster, and two spin 
  singlets with energy zero. We will consider large positive  The outline of this paper is as follows. In Section II 
  we will lay out the pseudo-spin-1/2 construction. In Sec- 