[DFTB-Plus-User] band alignment using DFTB calculations

[juhasz.g.aa]

Dear everybody,
I would like to compare the position of the bands in different semiconducting slabs 
I wonder if there is a way to estimate the work function or calculate (estimate) the vacuum level in DFTB calculations? Does it make any sense to calculate somehow an electrostatic potential from the calculated electronic density etc, or I should not rely on it that much?
Thank you for your advice,
Gergely Juhasz
Tokyo Institute of Technology, Japan