<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div> <blockquote type="cite"><div>Am 20.05.2026 um 11:37 schrieb 'Kit Joll' via cp2k <cp2k@googlegroups.com>:</div> <div><div style="border: 0px; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-variant-alternates: inherit; font-variant-position: inherit; font-width: inherit; font-size: 12pt; line-height: inherit; font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-optical-sizing: inherit; font-size-adjust: inherit; font-kerning: inherit; font-feature-settings: inherit; font-variation-settings: inherit; margin: 1em 0px; padding: 0px; vertical-align: baseline;">Dear CP2K developers,I am writing to ask for advice on a reproducible force-consistency issue that we see for bulk calcite, CaCO₃, using CP2K/Quickstep GPW.For a periodic cell at zero external electric field, the sum of the forces over all atoms should vanish to the accuracy of the force evaluation, even when the geometry is not relaxed. For the same 30-atom calcite frame, Quantum ESPRESSO gives a net force sum close to numerical zero, whereas CP2K gives a residual of order 1e-4 Ha/Bohr. In the attached representative CP2K output, for example, the force sum isFx = 3.49e-5 Ha/Bohr Fy = 1.13e-4 Ha/Bohr Fz = -5.39e-5 Ha/Bohrwith a total net force of about 1.30e-4 Ha/Bohr.This residual is small compared with typical individual atomic forces, but in our tests the same force-consistency issue contaminates finite-field response properties. Our intended workflow requires numerical derivatives of the forces with respect to external electric-field components, for example for atomic polar tensors and related higher-order response quantities. Other systems, including bulk water, transition-metal oxide/water interfaces, and cation solutions, behave well with the same CP2K finite-field workflow, so the problem appears to be specific to CaCO₃, or to this particular combination of pseudopotential, Gaussian basis, and GPW grid.The attached figure shows cutoff convergence for the energy per atom and for the zero-field net force sum. The top row is CP2K, using fixed REL_CUTOFF = 120 Ry and NGRIDS = 4 while varying the MGRID CUTOFF. The bottom row is Quantum ESPRESSO for the same structure. I include the QE result mainly as a translational-invariance sanity check, not as a strict force reference, since the pseudopotentials are different. QE gives a near-zero net force sum across the tested cutoffs, while CP2K retains a much larger residual. The CP2K energy also shows noticeable oscillations with increasing MGRID cutoff, which makes me suspect a grid/multigrid or Gaussian-grid-coupling contribution, although I am not sure.Could you advise which CP2K settings or tests would be most appropriate to diagnose this? In particular, I would be grateful for advice on whether this could be related to any of the following:<ul><li>the Ca/O/C pseudopotentials or basis sets, and their use with RPBE;</li><li>GPW multigrid effects or Gaussian-grid coupling;</li><li>Pulay/grid force terms;</li><li>CUTOFF, REL_CUTOFF, NGRIDS, or less obvious QS thresholds;</li><li>SCF/OT convergence settings.</li></ul>The attached input uses TZV2P-MOLOPT-PBE-GTH basis sets, GTH-PBE pseudopotentials, RPBE, EPS_SCF = 1e-9, OUTER_SCF EPS_SCF = 1e-9, EPS_DEFAULT = 1e-18, OT/CG with 3PNT line search, CUTOFF = 2500 Ry, REL_CUTOFF = 120 Ry, and NGRIDS = 4.I have attached a representative CP2K input/output pair for one frame. The same qualitative behaviour is observed across the other tested calcite geometries.Thank you very much for your time. I would really appreciate any suggestions for further tests or settings to try.Best wishes, Kit</div><div> </div> -- 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/f61ab10c-a7f9-425d-be7f-a6ab750b4a03n%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/f61ab10c-a7f9-425d-be7f-a6ab750b4a03n%40googlegroups.com</a>. <convergence_energy_and_force_sum.png><cp2k-output.out><cp2k-start.inp></div></blockquote></div> </body></html> -- You received this message because you are subscribed to the Google Groups "cp2k" group. 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