Hi Matthias Thanks a lot for your reply. Yes, a nonzero EPSILON is only make the updates ergodic but can not eliminate sign problem. I cannot understand what your said "one can get rid of a sign problem by changing the basis". Actually, I have no any experience to modify or add codes in original ALPS codes. So I have to ask for you help. Now I send my lattice model and Hamiltonian model files to you. The used model is a coupled two ladder lattice with five exchange coupling constants in Heisenberg model Humiltonian.You can find those details in attachments. Two attach files can be found in attachments including my_ctwoladder_lattice describles lattice model and my_models describles the used model Humiltonian. Your reply will be highly appreciated.
Forwordom