How to calculate the HOMO-LUMO gap in a QMMM simulation?

[jts2t... at gmail.com]

Dear researchers and developers,
I am a beginner in CP2K, and I major in computational mathematics. So the 
concepts about chemistry are new to me. I want to know how to get the 
HOMO-LUMO gap in a QMMM simulation. The setting in my input file is as 
follows

&PRINT
         &MO_CUBES
             NHOMO -1
             NLUMO -1
             WRITE_CUBE F
         &END MO_CUBES

which locates at the &DFT subsection. 

But the results seems unreasonable, the result of 2-QM-atom is even larger 
than the situation if I use full QM calculation. The whole system consists 
of 48 atoms. But if I specify 12 atoms as QM-atom, the result is less than 
the result of full QM calculation. So, is there any error in the setting ? 

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