Dear Adel, Thanks for answering me. I saw the FAQ and I try to reproduce for my case, but unfortunately didn't work. I changed the lead command: # Lead a direita (lead 1) lat_lead1 = kwant.lattice.square(C) sym_lead1 = kwant.TranslationalSymmetry((-C, 0)) lead1 = kwant.Builder(sym_lead1) lead1[(lat_lead1(0, 0))] = 0 lead1[lat_lead1(0,0), lat_lead1(1,0)] = -t syst[(a(0,0), lat_lead1(-1,0))] = -1 syst.attach_lead(lead1) kwant.plot(syst) plt.show() --------------------------------------------------------------------------- KeyError Traceback (most recent call last) <ipython-input-8-df1e018f6b56> in <module> 8 lead1[lat_lead1(0,0), lat_lead1(1,0)] = -t 9 ---> 10 syst[(a(0,0), lat_lead1(-1,0))] = -1 11 12 syst.attach_lead(lead1) ~\anaconda3\lib\site-packages\kwant\builder.py in __setitem__(self, key, value) 1160 func = (self._set_site if isinstance(sh, Site) 1161 else self._set_hopping) -> 1162 func(sh, value) 1163 1164 def _del_site(self, site): ~\anaconda3\lib\site-packages\kwant\builder.py in _set_hopping(self, hopping, value) 1136 # of identical sites. 1137 a2 = a = self.H[a][0] -> 1138 b2 = b = self.H[b][0] 1139 else: 1140 b2, a2 = sym.to_fd(b, a) KeyError: Site(kwant.lattice.Monatomic([[2.0, 0.0], [0.0, 2.0]], [0.0, 0.0], '', None), array([-1, 0])) If anyone can help me, i appreciate. Thaks. Gabriel Garcia