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

['Jürg Hutter' via cp2k]

Hi

1) You can try the Al (sr) basis from BASIS_MOLOPT. This text would give a hint if the UZH SCAN basis for Al is in some
    way corrupted. The SCAN/Hybrid basis sets are less texted than the PBE ones.
2/3) r2SCAN needs higher cutoffs than PBE. If the force wiggles are too large you can try GAPW and accurate_intrgration and
    finer_grid (in that order, accumulative)

regards
JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of liu wei <whisper256136 at gmail.com>
Sent: Monday, October 12, 2026 2:15 PM
To: cp2k
Subject: [CP2K:22433] Question about Al (GTH-SCAN-q3) forces with r2SCAN compared with all-electron / PAW results

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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