[Pw_forum] PBE Hybrid funcitonals and LDA pseudopotentials

[Gabriel Greene]

Hi,

I have been attempting to use the hybrid functional PBE0 to tune the bandgap of InGaAs, while using Virtual Crystal Approximation to obtain In(0.57)Ga(0.43)As. I initially tried mixing the PBE pseudos of Ga and In, but ran into a number of problems (unequal nqf mesh when mixing, and using ultrasoft pseudos with hybrid functionals).

I then tried the Norm-conserving LDA pseudos (which also have no nqf information to worry about) for mixing In and Ga (also using LDA for As pseudo, obviously). I then found I could tune the bandgap by specifying "PBE0" in the &SYSTEM card of the input file by changing the proportion of Fock exchange (alpha, or "exx_fraction" in funct.f90), while specifying LDA pseudos for each atomic species.

I am surprised this would work because my understanding is that LDA contains no exchange term, yet modifying the value of alpha has an effect on the bandgap just like if I had specified PBE pseudopotentials in the ATOMIC_SPECIES card. I initially set up this calculation as a test to just prove to myself that the results obtained using LDA pseudopotentials are independant of the amount of exchange, yet this is not so.

My understanding of hybrid functional implementation is obviously incomplete, so can someone help me understand these results?

Thank you very much for the help,

Gabriel Greene,
PhD student, Electronics Theory Group,
Tyndall National Institute,
Cork Ireland
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