[CP2K-user] [CP2K:22435] Question about Al (GTH-SCAN-q3) forces with r2SCAN compared with all-electron / PAW results

[liu wei]

Dear Prof. Jürg Hutter, 

Thank you very much for your quick and helpful reply. I ran the tests you 
suggested; the inputs, outputs and an
updated analysis script are in the attached cp2k_GTH-SCAN_report_v2.zip 
(new section A3 and an extended B1 in the README).
Please correct me if I have misread anything.

**1. Al: MOLOPT-SR basis**

I kept everything else fixed (GAPW + GAPW_ACCURATE_XCINT, all other 
elements TZV2P-MOLOPT-SCAN / GTH-SCAN) and changed
only the Al kind, on the original structure and the three MD frames. Al 
|ΔF| RMS against the all-electron Al
(meV/Å):

| Al basis + potential | initial | Al4 | Al5 | Al6 |
|---|---|---|---|---|
| TZV2P-MOLOPT-SCAN-GTH-q3 + GTH-SCAN-q3 (before) | 306 | 466 | 351 | 333 |
| DZVP-MOLOPT-SR-GTH-q3 + GTH-SCAN-q3 | 275 | 410 | 307 | 281 |
| TZV2P-MOLOPT-SR-GTH-q3 (BASIS_MOLOPT_UCL) + GTH-SCAN-q3 | 314 | 487 | 368 
| 350 |
| DZVP-MOLOPT-SR-GTH-q3 + GTH-PBE-q3 | 172 | 245 | 187 | 181 |

- With the SR basis the Al difference changes only a little (about −10 % 
with DZVP-SR, no change with TZV2P-SR), so it
  does not seem to come from the SCAN basis.
- Surprisingly to me, switching only the potential from GTH-SCAN-q3 to 
GTH-PBE-q3 removes about half of the difference,
  in the direction of the all-electron result, although the two parameter 
sets differ by less than 1 %. The trend on
  the Al–O pairs and on the relative energies of the Al4/5/6 frames is 
reduced in the same way, but not removed
  (about 180 meV/Å remains).

So the Al forces seem to be very sensitive to the potential parameters. 
Would you expect this for GTH-SCAN-q3, and
would you recommend a different approach for Al in oxides with r2SCAN, for 
example a potential with 2s2p in the
valence (I could not find one for SCAN in POTENTIAL_UZH), or simply the 
all-electron Al with GAPW?

**2/3. Zn grid sensitivity**

Following your suggestion, I added GAPW, GAPW_ACCURATE_XCINT and 
USE_FINER_GRID step by step (1100 Ry, 302 atoms,
4 rigid shifts). Zn force spread, |ΔF| RMS (meV/Å):

| Setting | Zn spread |
|---|---|
| GPW | ~390 |
| GAPW + GAPW_ACCURATE_XCINT | 37 |
| GPW + USE_FINER_GRID | 28–32 |
| GAPW + GAPW_ACCURATE_XCINT + USE_FINER_GRID | 0.7–0.8 |

- The full combination works very well: the Zn noise drops below 1 meV/Å. 
Thank you for this.
- I also changed only the Zn basis to TZVP-MOLOPT-SR-GTH and the potential 
to GTH-PBE-q12; the noise stayed at
  ~390 meV/Å, so with these choices it seems to come from r2SCAN on the 
grid rather than from the SCAN basis or potential.

May I ask one more question on this? Would it be possible, or sensible at 
all, to reduce this grid sensitivity on the
side of the potential or the basis set, for example a Zn potential/basis 
built to be smoother on the grid for
meta-GGAs? Since the SR basis and the GTH-PBE potential did not change it, 
I do not know whether this is something the
potential or basis can address.

Thank you again for your time and help.

Best regards,
Wei LIU


在2026年10月12日星期一 UTC+9 22:23:26<Jürg Hutter> 写道：

> 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: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of liu 
> wei <whispe... 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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