[QE-users] LIBXC versus internal QE functionals

[Abdesalem Houari]

Dear All,
I wanted to do a calculation test using PBE-functional from LIBXC and compare it to the internal QE one. As a test, I opted for body centered cubic (bcc) iron.
I'm a bit puzzled by the differences in the results.
- First, for PBE from LIBXC, following the QE user-guide on how to use LIBXC, I put the following input: input_dft='XC-000I-000I-101L-130L-000I-000I'. I obtained a total energy value of -39.08694619 Ry.

- Then, for QE internal PBE functional, after having a look to Modules/funct.f90 file, I deduced the PBE should be forced from the input as follow: input_dft='XC-000I-000I-003I-004I-000I-000I'. Here I was quite surprised  obtaining a different value of the total energy  -31.22123378 Ry... It seems like it is not the correct PBE ?

- When I just use NC-PP (Fe.pbe-mt_fhi.UPF) without forcing the input_dft, I get a similar result to LIBXC-PBE ... The same result is obtained also when I forced the input_dft, but including LDA-exchange (Slater)+correlation (PW) i.e. input_dft='XC-001I-004I-003I-004I-000I-000I'

I'm really confused ! I would be grateful if some could give a bit of explanation, on how the input_dft works exactly ? especially with respect to the Module/funct.f90 file which gives all the internal functionals.  


I would also ask for another issue. It is explained in the PW manuel that input_dft overrides the PP. Normally, if I'm not wrong, whatever the kind of the PP given in the input file (PBE or other; norm-conserving or ultra-soft or PAW potential), the code will discard it and does not look at it at all when input_dft is forced.Here also I'm confused, I forced the input_dft as: input_dft='XC-001I-004I-003I-004I-000I-000I', with two different PP (Fe.pbe-mt_fhi.UPF and Fe.pbe-spn-rrkjus_psl.0.2.1.UPF); and the total energies are completely different !  

Thanks a lot in advance

Sincerely yours.

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Dr. Abdesalem HOUARI-------------------------------------------------------------------------------------------
Department of physics, Theoretical Physics Laboratory
University of Bejaia-06000. Algeria.
E-mail: abdeslam.houari at univ-bejaia.dz & habdslam at yahoo.fr
https://sites.google.com/site/houariabdeslam/homepage
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