For a device application, you need the dielectric function. The experimental is the best. It includes all quantum effects.
You can calculate it in a model, or you can use a phenomenological model, for example describing interband transition with a Lorentz function.
To calculate the dielectric function from the band structure, you need in addition to the structure itself, the matrix elements of transitions between different states. Then calculate the appropriate sum. It is a lot of work to perform for the device application with little result.