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 ?? | [( 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 | | Offdiagonal spin correlations[( 0 ) -- ( 3 )] | -0.03664352593820266 | | Offdiagonal spin correlations[( 0 ) -- ( 4 )] | 0.002203905249778245 | | Offdiagonal spin correlations[( 1 ) -- ( 3 )] | 0.0331275686776057 | | Offdiagonal spin correlations[( 1 ) -- ( 4 )] | -0.007827950318578362 | | Offdiagonal spin correlations[( 0 ) -- ( 4 )] | -0.04668994676799755 | | Offdiagonal spin correlations[( 0 ) -- ( 5 )] | 0.01338526650557481 | | Offdiagonal spin correlations[( 1 ) -- ( 4 )] | 0.05418339350735944 | | Offdiagonal spin correlations[( 1 ) -- ( 5 )] | -0.02572536103232437 | | Offdiagonal spin correlations[( 0 ) -- ( 5 )] | 0 | | Offdiagonal spin correlations[( 0 ) -- ( 6 )] | 0 | | Offdiagonal spin correlations[( 1 ) -- ( 5 )] | 0.01195256946918583 | | Offdiagonal spin correlations[( 1 ) -- ( 6 )] | 0.2499999999999999 | | Offdiagonal spin correlations[( 0 ) -- ( 6 )] | 0.01194311395908962 | | Offdiagonal spin correlations[( 0 ) -- ( 7 )] | -0.1411693721423247 | | Offdiagonal spin correlations[( 1 ) -- ( 6 )] | -0.02576129524991342 | | Offdiagonal spin correlations[( 1 ) -- ( 7 )] | 0.05444259724942019 | | Offdiagonal spin correlations[( 0 ) -- ( 7 )] | 0.01371611664136257 | | Offdiagonal spin correlations[( 0 ) -- ( 8 )] | -0.09445304446831181 | | Offdiagonal spin correlations[( 1 ) -- ( 7 )] | -0.01667478948175052 | | Offdiagonal spin correlations[( 1 ) -- ( 8 )] | 0.03335169908714469 | | Offdiagonal spin correlations[( 0 ) -- ( 8 )] | 0.002207364935941955 | | Offdiagonal spin correlations[( 0 ) -- ( 9 )] | -0.03640998935519778 | | Offdiagonal spin correlations[( 1 ) -- ( 8 )] | 0.00226127678640559 | | Offdiagonal spin correlations[( 1 ) -- ( 9 )] | 0.03307792417161022 | | Offdiagonal spin correlations[( 0 ) -- ( 9 )] | -0.008440335344440209 | | Offdiagonal spin correlations[( 0 ) -- ( 10 )] | -0.04669087903965578 | | Offdiagonal spin correlations[( 1 ) -- ( 9 )] | 0.01383912437540501 | | Offdiagonal spin correlations[( 1 ) -- ( 10 )] | 0.05428792435810501 |