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 -- You received this message because you are subscribed to the Google Groups "cp2k" group. To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:cp2k+unsubscribe@googlegroups.com">cp2k+unsubscribe@googlegroups.com</a>. To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/189080b0-6d63-4515-9a04-191b65347daan%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/189080b0-6d63-4515-9a04-191b65347daan%40googlegroups.com</a>.