[CP2K-user] [CP2K:22433] Question about Al (GTH-SCAN-q3) forces with r2SCAN compared with all-electron / PAW results

[liu wei]

Dear CP2K developers and users,

I am a student working on ab initio MD of a ZnO–P2O5–Al2O3 glass (302 
atoms, Γ point) with r2SCAN-D3(BJ),
the GTH-SCAN potentials and the MOLOPT-SCAN basis sets (CP2K 2026.2). While 
I was checking my forces against VASP,
I noticed a difference on Al that I do not understand, and I would be very 
grateful for your advice. I may well be
missing something in my setup, so please correct me if so.

I have attached all inputs and outputs, a short analysis script (numpy 
only) and a README with the full tables
(cp2k_GTH-SCAN_report.zip).

**1. Al forces**

On one 302-atom structure I obtained the following force differences (|ΔF| 
RMS per element, meV/Å):

| A − B | Zn | O | P | Al |
|---|---|---|---|---|
| CP2K (GPW, TZV2P-MOLOPT-SCAN, GTH-SCAN, 800 Ry, FINER) − VASP (PAW 
O_h/P_h/Zn/Al_sv_GW, 1300 eV) | 41 | 55 | 61 | 320 |
| CP2K GAPW, Al → ALL + pob-TZVP-rev2 − CP2K GAPW, Al GTH-SCAN-q3 
(everything else identical) | 3 | 40 | 11 | 306 |
| CP2K GAPW with all-electron Al − VASP Al_sv_GW | 58 | 39 | 61 | 62 |

- For Zn, O and P the two codes agree to about 40–60 meV/Å, and the 
pressure agrees to within 1 kbar, but for Al the
  difference is about 300 meV/Å.
- When I replaced only the Al GTH-SCAN-q3 by an all-electron Al in GAPW, 
most of this difference (about 94 %)
  disappeared in my test.
- I tried to check a few other possible reasons: USE_FINER_GRID, GPW vs 
GAPW (0.6 meV/Å
  on Al), and the frozen 2s2p core on the VASP side (Al vs Al_sv_GW changes 
the Al forces by 69 meV/Å, in the other
  direction). None of them seemed to explain it, but I am not sure whether 
I checked them in the right way.
- I also looked at three MD snapshots with 4-, 5- and 6-coordinated Al. On 
all 6 Al the difference seems to follow the
  same trend: compared with the all-electron Al, GTH-SCAN-q3 appears to 
pull the short Al–O bonds (< 1.85 Å) in
  and push the longer Al–O contacts (2.1 Å and beyond) out; between 1.85 
and 2.1 Å the sign varies. Here I project
  ΔF = F(all-electron) − F(GTH-SCAN-q3) on each Al–O pair as (ΔF_O − 
ΔF_Al)·u, with u the unit vector from Al to O.
  A positive value means the all-electron force along the pair is more 
repulsive than the GTH-SCAN-q3 one (so
  GTH-SCAN-q3 pulls the pair together), and a negative value means it is 
more attractive (GTH-SCAN-q3 pushes the
  pair apart). These are differences, not the total forces. For example, I 
get +812 meV/Å for a bond of 1.66 Å and
  −436 meV/Å for a contact of 2.10 Å.
- One limitation I am aware of: the all-electron run also uses a different 
Al basis (pob-TZVP-rev2), so I could not
  separate the effect of the potential from that of the basis. Changing 
only the MOLOPT basis (TZVP → TZV2P) moved
  the Al forces by 87 meV/Å, so I guessed the potential plays the larger 
role, but I am not confident about this.

**2. Zn (a smaller question)**

With GPW, r2SCAN + TZVP-MOLOPT-SCAN + GTH-SCAN-q12 gave a Zn force spread 
of about 390 meV/Å RMS over 4 rigid shifts
of the same structure, while PBEsol + TZVP-MOLOPT-SR + GTH-PBE-q12 changed 
by only about 2 meV/Å when the FFT grid
changed (1100 vs 1350 Ry). USE_FINER_GRID reduced the r2SCAN spread to 
about 30 meV/Å, which works well for me.
I did not separate the effects of the functional, the basis and the 
potential, so I am not sure where this comes from.

**My questions**

1. Could you help me understand what might cause the Al difference? Is it 
something one would expect with
   GTH-SCAN-q3 (for example, because it was fitted for SCAN rather than 
r2SCAN), or is there likely a problem in my
   setup or in the way I compared the results?
2. For Al in oxides with SCAN/r2SCAN, would you recommend a different 
potential or basis (for example, one with
   2s2p in the valence), or another way to check this more carefully?
3. For Zn with r2SCAN, is this grid sensitivity expected, and is there 
anything you would recommend besides
   USE_FINER_GRID?

I would be happy to run any additional tests you suggest. Thank you very 
much for your time and for developing
CP2K.

Best regards,
Wei LIU

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