Help tuning HSE exact-exchange fraction for AlN band gap in CP2K

[Asif Billah]

Hello CP2K community,

I am calculating the bulk AlN band gap using CP2K 2024.1 with HSE, ADMM, a 72-atom 3x3x2 wurtzite supercell, and Gamma point sampling. My target band gap is approximately 6.1 eV.

Using standard diagonalization, Broyden mixing, ADMM_PURIFICATION_METHOD NONE, and alpha=0.15, I obtained a converged gap of:

Band gap = 5.267211 eV

However, when I used alpha=0.20, the SCF initially approached about 5.7 eV but later diverged catastrophically. A retry with DIRECT_P_MIXING and a level shift produced a NaN in the first SCF step.

Could you please advise: What is the recommended stable procedure or strategy so that the calculated bandgap reaches 6.1 eV using HSE calculations with ADMM? 

Any guidance would be greatly appreciated.

Thank you.