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## Main Question or Discussion Point

Why is minus sine in definition of hamiltonian

[tex]H=-\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]

Why not?

[tex]H=\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]

[tex]H=-\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]

Why not?

[tex]H=\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]