I'm doing research on the properties of Germanene nanoribbons and encountered a result that my advisor found questionable. For the four atom AC nanoribbon, the bandgap is roughly 0.8eV, and we're trying to shrink that. However when I increase the width by a single atom, it decreases to 0.08eV. My advisor says this is too small to be accurate, and mentioned using the quantum mechanical model in ATK. It's not a problem I've been able to solve by hand, so what options are available for a more comprehensive band structure analysis? Where can I find information on adding the quantum mechanical model to my calculation, and does this bandgap seem reasonable? Any help would be appreciated.

For my bandstructure calculator, in the new calculator box I used the Extended Huckel with no SCF iteration and dirichlet boundary conditions for the regions bounded by hydrogen. For the bandstructure I used the simple bandstructure box and 200 points per segment. I shared pictures of my results below. I used geometry optimization to obtain the buckling, and share pictures of both the 4 atom nanoribbon and 5 atom one, with bandstructures for both.