The germanium band structure is a tough one, even for pure Ge :) After performing a few different test calculations, I suggest you try out the newly implemented PseudopotentialProjectorShift (PPS) method along with SG15 pseudopotentials, see http://docs.quantumwise.com/manuals/Introduction.html#atk-2016 (http://docs.quantumwise.com/manuals/Introduction.html#atk-2016) and http://docs.quantumwise.com/manuals/Types/PseudoPotentialProjectorShift/PseudoPotentialProjectorShift.html#NL.Calculators.DensityFunctionalTheory.LCAOCalculator.BasisSet.PseudoPotentialProjectorShift (http://docs.quantumwise.com/manuals/Types/PseudoPotentialProjectorShift/PseudoPotentialProjectorShift.html#NL.Calculators.DensityFunctionalTheory.LCAOCalculator.BasisSet.PseudoPotentialProjectorShift), and http://docs.quantumwise.com/manuals/ATKDFT.html#sec-sg15 (http://docs.quantumwise.com/manuals/ATKDFT.html#sec-sg15).
For germanium, the following projector-shift parameters should work well:
projector_shift = PseudoPotentialProjectorShift(s_orbital_shift=15.0*eV,
p_orbital_shift=0.2*eV,
d_orbital_shift=-2.0*eV)
See attached script on how to use the PPS method. The script relaxes the Sn1Ge15 bulk and performs band structure + DOS analysis. A fairly small gap opens up at the Gamma points, see attahced PNG.
BTW: I can think of 2 reasons why this system may be so hard:
1) germanium is hard to describe with simple methods, such as GGA and MGGA. Perhaps specialized methods must be used, e.g. PPS or HSE.
2) perhaps relaxation effects are very important, i.e., perhaps the lattice constant needs to be just right in order to get the band gap. Just a thought...