Strange behavior of current calculation

[ziand]

We are calculating I-V curves non-selfconsistently form one single transmission spectrum. Details are not interesting. For simplicity: suppose the spectrum is from -1eV to +1eV and constant (e.g. T(E)=1 for all E). Now we calculate the current from zero bias up to 2 Volt (!) at finite temperature (e.g. 300 K). Clearly the bias window coverage gets worse when approaching 2 Volt and the situation is better for lower temperatures.

Now the point I want to know is: What does ATK assume for the part of the transmission spectrum that is outside the window (the "missing data points" for E < -1eV and E > +1eV). Does it just drop it and assume T(E)=0 there or may it be that it uses T(E<-1eV) = T(-1eV) and T(E>+1eV) = T(+1eV) or something else?

The reason why I ask is the following. We did such a calculation (but for a real transmission spectrum which is not constant of course) and got an unrealistically strong increase in the current when approaching the boundaries of the transmission. We repeated the calculation "by hand", doing a simple manual integration. The ATK-current curve and the manual calculated one coincide for small and medium bias but there is a STRONG at higher values (first picture). I have to admit that our simple integeration scheme takes T(E)=0 outside the given energy range which is surely a bit unrealistic, but the results of ATK seem much to large. The ATK-curve is clipped but goes up by around an order of magnitude.

We repeated the procedure for much lower temperature and results get better (the point where both curves split shifts to higher voltages). Could you comment on what ATK is doing here?

By the way: Conductance calculations look odd, too (see second picture).

We can provide a minimal test script if needed.

[kstokbro]

We assume that the transmission coefficient is constant outside the transmission window, thus, if this is not a good approximation for your system you must include more energy points.

The conductance looks strange, please  try to redo the conductance calculation with 12.2 since we have changed the integration routine, and it should be better now. 12.2 is backwards compatible, thus, you do not need to redo the transmission calculation.

If it still does not look correct please attach your input or nc file ( or send to info@quantumwise.com) so we can check it.