Fermi energy for one composite system

[vaspfonda]

Dear all:

I have one question concerning the Fermi energy for a composite system, such as one atomic layer system on top of SiO2 substrate.
The free standing atomic layer has a band gap, so does the SiO2 substrate. But when I put two subsystems together and did VASP
calculation, I found the Fermi level for this new van der Waals (vdW) stacking system lies in the valence band of atomic layer, which
is quite strange to me.

So my questions are:
(1) Is this Fermi energy reliable for such a vdW system? Or, is there an artifact for this kind of calculation?
(2) How to evaluate the energy barrier for electron hopping from one subsystem (atomic layer) to another
subsystem (SiO2)?

Thanks for your participation!

VASPfonda