Dear Matthias:

  I begin with the Hamiltonian for a chain with spins S=1/2 ,

    

   use periodic boundary conditions ,the total number of sites is N=20. calculate correlation :

  Offdiagonal spin correlations  has problem,

    why 0-0  and  0-1   offdiagonal spin  correlations  value is 0   ??

   

Offdiagonal spin correlations[( 0 ) -- ( 0 )]	0
Offdiagonal spin correlations[( 0 ) -- ( 1 )]	0
Offdiagonal spin correlations[( 1 ) -- ( 0 )]	0.2499999999999999
Offdiagonal spin correlations[( 1 ) -- ( 1 )]	0.01195256946918583
Offdiagonal spin correlations[( 0 ) -- ( 1 )]	-0.1410917216909388
Offdiagonal spin correlations[( 0 ) -- ( 2 )]	-0.02578705926914329
Offdiagonal spin correlations[( 1 ) -- ( 1 )]	0.05437436640436828
Offdiagonal spin correlations[( 1 ) -- ( 2 )]	0.01374443422199295
Offdiagonal spin correlations[( 0 ) -- ( 2 )]	-0.04722700778251831
Offdiagonal spin correlations[( 0 ) -- ( 3 )]	-0.01673969792911871
Offdiagonal spin correlations[( 1 ) -- ( 2 )]	0.06699300813943075
Offdiagonal spin correlations[( 1 ) -- ( 3 )]	0.002194337470049455