[Pw_forum] about energy gap

[sun jason]

Dear all,

I'm a freshman in computational materials, I'm puzzled by a simple question 
about energy gap, when I calculated the C2-BN system which was construtured 
by mixing cubic BN and diamond. 

Well, the computed LDA band gap of c-BN and diamond is around 4.5 and 4.1 
eV, respectively. Therefore, by mixing two compounds with such a wide band 
gap (roughly of the order of 4
eV) one might expect to get a BC2N compound with an Eg between 4.5 and 
4.1eV. Hence, why is our calculated Eg (about 1.0 to 2.0 eV) is much lower 
than expected for a C2-BN system? 

the unit cell I calculated is crystallined in diamond-like structure and 
the atoms are sp3 hybridization.

Thank you very much!