Different ways to run PBE50?

[Mikhayla Clothier]

Hello,

I would like to use the PBE50 functional. I followed the tutorial provided on the website (https://www.cp2k.org/exercises:2017_uzh_cp2k-tutorial:hybrid) where the PBE50 functional is created by manually tuning the PBE and HF contributions,

    &XC
        &XC_FUNCTIONAL
            &PBE
                SCALE_X 0.50
                SCALE_C 1.0
            &END PBE
        &END XC_FUNCTIONAL
        &HF
            FRACTION 0.50
        &END HF
    &END XC

However, within the manual, there seems to be a separate way to call the PBE50 functional from the LibXC library by simply saying

    &XC
        &XC_FUNCTIONAL
            &HYB_GGA_XC_PBE50
            &END HYB_GGA_XC_PBE50
        &END XC_FUNCTIONAL
    &END XC

without the need to manually specify the HF contribution. I ran both approaches with a single water molecule to compare, and I got different energies. Also, just using the HYB_GGA_XC_PBE50 approach did not result in the "Hartree Fock exchange energy" being printed in the output.

Which method is the correct approach? If its the first, how does the HYB_GGA_XC_PBE50 functional work?  I am using the 2022.1 version of CP2K with the 4.1.4 version of OpenMPI.

I've attached the output files to this post. The one for manually setting PBE50 is named
"water_manual.out" and the one where I just set the functional as HYB_GGA_XC_PBE50 is called "water_hyb.out." If I need to upload my input files as well, please let me know!

Thank you!

Mikhayla Clothier

[Frederick Stein]

Dear Mikhayla,

The first approach is correct. But you can make the second approach running by adding the HF section. The LibXC-related sections only affect the DFT part, not the HF part. It is always up to you to set it up because different systems require different setups regarding EPS_SCHWARZ, EPS_SCHWARZ_FORCES, MAX_MEMORY, CUTOFF_RADIUS etc.

HTH,

Frederick

[Mikhayla Clothier]

Dear Frederick,

Thank you so much for your response! I'll set it manually from here on out.

Mikhayla Clothier