Electronic band structure of semiconductor alloys: From ab initio to k·p via computational alchemy, on example of Ge1-xSnx alloy

•A computationally effective band structure model for semiconductor alloys.•The state of the art, supercell based ‘ab initio’ calculations as reference.•The computational alchemy as an intermediate step to fit the 30-band kp model.•Band structure reproduced in the full composition range and in the full Brillouin Zone.•An advantage of the method shown on example of the material gain in GeSn alloy.