Field emission calculations typically assume a "one-probe" geometry rather than the two-probe boundary conditions used in the design of ATK. One could try to mimic a one-probe by using a large empty central region, but the calculations may not be very accurate since the tunneling gap may have to be so large (to avoid direct tunneling) that the left and right surfaces have no basis function overlap, in which case the calculations probably break down. One way to overcome that would be to introduce vacuum basis sets (dummy atoms), and that is something that might appear in future versions of ATK.
