As a lead I want to use the left boundary atoms of graphene (please see attached photo). I use class kwant.builder.BuilderLead(builder, interface). I define a builder in the following way: #The tight-binding system of a left lead. left_lead = kwant.Builder() left_lead[lat(- 90.26501 , 13.2107 )] = 1.5 left_lead[lat(- 91.60269 , 12.67463 )] = 1.5 left_lead[lat(- 91.59045 , 11.58774 )] = 1.5 left_lead[lat(- 90.26237 , 11.08828 )] = 1.5  etc. And add hoppings left_lead[lat(- 91.60269 , 12.67463 ) , lat(- 90.26501 , 13.2107 )] = 2j left_lead[lat(- 91.59045 , 11.58774 ) , lat(- 91.60269 , 12.67463 )] = 2j left_lead[lat(- 90.26237 , 11.08828 ) , lat(- 91.59045 , 11.58774 )] = 2j left_lead[lat(- 90.26083 , 10.04835 ) , lat(- 90.26237 , 11.08828 )] = 2j etc The same way I define the interface (which called left_sequence) but without the hoppings. left_sequence = kwant.Builder() left_sequence[lat(- 88.99753 , 12.6423 )] = 1.5 left_sequence[lat(- 89.0014 , 11.61135 )] = 1.5 left_sequence[lat(- 88.9998 , 9.535691 )] = 1.5 left_sequence[lat(- 89.00571 , 8.504004 )] = 1.5 left_sequence[lat(- 88.99628 , 6.424798 )] = 1.5 etc l_lead = kwant.builder.BuilderLead(left_lead , left_sequence) sys .leads.append(l_lead) kwant.plot(sys) While I try to plot sys I have encountered an error:  IndexError: Index 0 out of range (-0 <= index < 0) in dimension 0. I think that this error may appear because I incorrectly pass parameters builder and interface to a class. С уважением, Татьяна Григоренко