Hello All, I want to obtain the hamiltonian for my system, for which I am using the *sys.hamiltonian_submatrix()* command, but I get the following error. *UserCodeError: Error occurred in user-supplied value function "onsite".* Below is the code that I am using. *lat = kwant.lattice.honeycomb(a = a)* *p, q = lat.sublattices* *def make_system():* * def hopping(sitei, sitej, phi, salt):* * xi, yi = sitei.pos* * xj, yj = sitej.pos* * return -t * np.exp(-0.5j * phi * (xi - xj) * (yi + yj))* * def onsite(site, phi, salt):* * x, y = site.pos* * return 0* * def central_region(pos):* * x, y = pos* * return abs(x) < 2*a and abs(y) < 3*a* * sys = kwant.Builder()* * sys[lat.shape(central_region, (0, 0))] = onsite* * sys[lat.neighbors()] = hopping* * sym = kwant.TranslationalSymmetry((-a, 0))* * lead = kwant.Builder(sym)* * lead[lat.shape(lambda s: abs(s[1]) < 2*a, (0, 0))] = 0* * lead[lat.neighbors()] = hopping* * sys.attach_lead(lead)* * sys.attach_lead(lead.reversed())* * return sys.finalized()* *sys = make_system()* *sparse_mat = sys.hamiltonian_submatrix() ----> Throws the error* What confuses me is that the code works fine (when I do further calculations on sys) if I comment the *hamitonian_submatrix()* command but throws up the error otherwise. That made me think that the code itself should be correct, but I could not figure out why that line gives an error. I would be glad if someone can help me out with the same. Any help will be much appreciated! Sincerely, Shivang Agarwal -- *Shivang Agarwal* Junior Undergraduate Discipline of Electrical Engineering IIT Gandhinagar Contact: +91-9869321451