Dear all, Please help me in correcting the following program which I tried to mimic a portion of the program on transmission through a quantum well. I tried the following: import kwant import cmath lat=kwant.lattice.square(1) sys=kwant.Builder() def potential(site,pot): (x,y)=site.pos if 10 < x< 20: return pot else: return 0 def onsite(site,pot=0): return potential(site,pot) print potential(12,0) Error shown : (x,y)=site.pos; there is no attribute in int (x,y)= site.pos It is strange that in the original program of the quantum well, I also could not explain why that program run successfully! because there also no such "pos" attribute is defined anywhere. Please bear with my misunderstanding and rectify me with brief tutorial. K.K.Ghosh mail: kk_ghosh@rediffmail.com