Dear Thomas, <div><br /></div><div>Thank you very much for fixing this issue so quickly. I'm recompiling now and then will attempt to reproduce your findings. I've also seen you've updated the APT tests, so I am presuming you don't need a minimal regtest example anymore? If you do, please let me know and I'll create something. </div><div><br /></div><div>Once again, thank you for your quick replies and help. </div><div><br /></div><div>All the best,</div><div>Kit<br /><br /></div><div class="gmail_quote"><div dir="auto" class="gmail_attr">On Wednesday, May 27, 2026 at 11:27:48 PM UTC+1 Thomas Kühne wrote:<br/></div><blockquote class="gmail_quote" style="margin: 0 0 0 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div style="line-break:after-white-space"><div>Dear Kit,</div><div><br></div><div>thank you for the additional finite-field checks. Your observation was exactly the useful diagnostic: the </div><div>residual is odd in the applied field, proportional to the field strength, and parallel to the field direction. </div><div>This points to the periodic Berry-phase electric-field force contribution rather than to the GAPW_XC </div><div>grid issue.</div><div><br></div><div>I have prepared a CP2K patch that enforces the acoustic sum rule on the isolated periodic electric-field </div><div>force contribution for neutral periodic cells. With your +1.25e-4 a.u. calcite case, the force sum changes </div><div>from</div><div><br></div><div> (-6.11e-9, 7.87e-9, 1.02476e-4) Ha/Bohr</div><div><br></div><div>to</div><div><br></div><div> (-6.11e-9, 7.87e-9, -3.02e-9) Ha/Bohr,</div><div><br></div><div>with the total force sum reduced to about 1.0e-8 Ha/Bohr.</div><div><br></div><div>The energy is unchanged within numerical noise. This confirms that the remaining finite-field force-sum </div><div>residual was a center-of-mass component of the periodic electric-field force, distinct from the original </div><div>XC/grid force issue.</div><div><br></div><div>A smaller reproducer would still be useful for adding a lightweight regression test, since the full calcite case </div><div>is too expensive for the regular CP2K test suite.</div><div><br></div><div>Best,</div><div>Thomas</div><div><br><blockquote type="cite"></blockquote></div></div><div style="line-break:after-white-space"><div><blockquote type="cite"><div>Am 27.05.2026 um 14:10 schrieb 'Kit Joll' via cp2k <<a href data-email-masked rel="nofollow">cp...@googlegroups.com</a>>:</div><br></blockquote></div></div><div style="line-break:after-white-space"><div><blockquote type="cite"><div><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">Dear Thomas,</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">Thank you again for the helpful suggestions. I have now repeated the finite-field calculation with both signs of the field and with the field magnitude halved.</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">All four calculations used the same input settings, cell, coordinates, GAPW_XC / GAPW_ACCURATE_XCINT setup, XC grid settings, basis sets and pseudopotentials. The only changes were the PERIODIC_EFIELD INTENSITY and POLARISATION. The effective field is along z, with E_z = INTENSITY * POLARISATION_z. I have attached the four corresponding input/output pairs. The attachment filenames indicate the actual field values (the PROJECT_NAME inside the input files is just a leftover label).</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">The resulting force sums are:</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">E_z / a.u. Sum F_x Sum F_y Sum F_z (Sum F_z) / E_z</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">-2.5e-4 -6.1085e-09 7.8702e-09 -2.04936242e-04 0.819744968</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">-1.25e-4 -6.1090e-09 7.8708e-09 -1.02473941e-04 0.819791528</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">+1.25e-4 -6.1100e-09 7.8720e-09 1.02475947e-04 0.819807576</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">+2.5e-4 -6.1104e-09 7.8725e-09 2.04962359e-04 0.819849436</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">So the absolute z-component of the force-sum residual approximately halves when the field is halved, while the ratio Sum F_z / E_z remains essentially constant at about 0.8198. The residual also changes sign when the field direction is reversed. The transverse components remain at the zero-field level, around 1e-8 Ha/Bohr.</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">The central finite-difference slopes are very similar for the two field magnitudes:</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">[ F_z(+1.25e-4) - F_z(-1.25e-4) ] / [2 * 1.25e-4] = 0.819799552</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">[ F_z(+2.5e-4) - F_z(-2.5e-4) ] / [2 * 2.5e-4] = 0.819797202</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">This seems to confirm your suggested diagnostic: the remaining force-sum residual is odd in the field, proportional to the applied field, and parallel to the field direction. It therefore looks distinct from the original XC/grid force issue.</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">Would you recommend any further diagnostic at this point, or would it be useful for me to prepare a more minimal test case for the periodic-electric-field force contribution?</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">All the best,</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none;float:none;display:inline!important">Kit</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><div dir="auto" class="gmail_attr">On Tuesday, May 26, 2026 at 7:15:40 PM UTC+1 Thomas Kühne wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="line-break:after-white-space"><div>Dear Kit,</div><div><br></div><div>I agree with your interpretation. The important point is that the residual is now almost entirely parallel to the applied periodic field. In your z-field calculation,</div></div><div style="line-break:after-white-space"><div><br></div><div>FORCES| Sum -6.11045615E-09 7.87254948E-09 2.04962359E-04</div><div><br></div></div><div style="line-break:after-white-space"><div>the x and y components remain at the zero-field accuracy, whereas the z component is of the order of the applied field strength. This strongly suggests that </div><div>we are no longer looking at the same XC/grid egg-box issue that was cured by GAPW_XC + GAPW_ACCURATE_XCINT. Rather, the remaining term seems </div><div>to originate from the finite-field/Berry-phase force contribution itself.</div><div><br></div><div>For a neutral periodic system I would still expect the total force to vanish, also in a homogeneous Berry-phase electric field, up to numerical noise. The </div><div>residual is also not simply a physical net-charge force. The system is neutral in terms of the GTH valence charges, and the residual is approximately 0.82 </div><div>times the field strength rather than an obvious integer-charge contribution.</div><div><br></div><div>I do not think there is a Wannier-localization setting that should fix this. The PERIODIC_EFIELD implementation uses the Berry-phase formulation directly; </div><div>the printed Wannier/localization controls are not convergence parameters for this force term. Likewise, increasing the XC fine grid should only affect the </div><div>ordinary XC/grid part. Since the transverse components are already small, and the residual tracks the field direction, I would not expect FINE_GRID_FACTOR </div><div>alone to remove it completely.</div><div><br></div><div>Could you please do two quick checks, if convenient?</div><div><br></div><div>1. Repeat the same calculation with the field reversed, i.e. INTENSITY -2.5e-4 or POLARISATION 0 0 -1.</div><div>2. Repeat with half the field strength, e.g. 1.25e-4 a.u.</div><div><br></div><div>If the force-sum residual changes sign with the field and scales linearly with its magnitude, that would be very strong evidence that the issue is in the periodic </div><div>electric-field force contribution rather than in the GAPW/XC integration.</div><div><br></div><div>At first sight, finite-field force code path looks like a missing or imperfect cancellation between the ionic and electronic Berry-phase force terms under a uniform </div><div>translation. In other words, it may be a real CP2K bug or at least a limitation of the present PERIODIC_EFIELD force implementation, rather than an </div><div>input-convergence issue.</div><div><br></div><div>As a practical workaround for testing internal forces, subtracting the average force from all atoms would remove the spurious center-of-mass acceleration, but </div><div>I would not present that as a formally clean solution until we understand the origin of the term.</div><div><br></div><div>Best wishes,</div><div>Thomas</div><div><br><blockquote type="cite"></blockquote></div></div><div style="line-break:after-white-space"><div><blockquote type="cite"><div>Am 26.05.2026 um 16:35 schrieb 'Kit Joll' via cp2k <<a rel="nofollow">cp...@googlegroups.com</a>>:</div><br></blockquote></div></div><div style="line-break:after-white-space"><div><blockquote type="cite"><div><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Dear Thomas,</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Thank you again for your help. I was able to reproduce your zero-field results to approximately the same accuracy with a master version cloned on Friday 22nd May. I presume the remaining small differences are due to build/library differences.</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">However, while the zero-field force sum is now well converged, I am not seeing the same behaviour once a finite periodic electric field is applied.</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">For example, using</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">METHOD GAPW_XC<br>GAPW_ACCURATE_XCINT<br>USE_FINER_GRID T<br>FINE_GRID_FACTOR 2.0</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">together with a periodic Berry-phase electric field of intensity 2.5e-4 a.u. along z, the force sum becomes</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">FORCES| Sum -6.11045615E-09 7.87254948E-09 2.04962359E-04<br>FORCES| Total atomic force 2.04962360E-04</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">So the transverse components remain well converged, but the component along the applied field direction returns to the order of 1e-4 Ha/Bohr. I see similar behaviour when applying the field along other directions. The issue also persists when increasing FINE_GRID_FACTOR from 2 to 8: the force-sum error remains orders of magnitude larger along the field axis (however, all components sum do decrease by ~1 order of magnitude relative to FINE_GRID_FACTOR 2). </p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Do you know whether there are additional grid, Berry-phase, or Wannier-related settings that need to be converged for finite-field force calculations? Or is there another setting that should be used together with GAPW_XC and GAPW_ACCURATE_XCINT in the presence of a periodic electric field?</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Please find attached a minimal input (start.inp) /output (output.out) pair that reproduces the issue.</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">Thank you again for your help and advice.</p><p style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none">All the best,<br>Kit</p><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><div dir="auto" class="gmail_attr">On Thursday, May 21, 2026 at 10:48:00 AM UTC+1 Kit Joll wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div><span dir="auto"></span></div><div><span dir="auto"><div dir="auto"><p>Dear Thomas,</p><p>Many thanks for the very quick and detailed reply! This is extremely helpful, and your diagnosis makes a lot of sense given the behaviour we were seeing.</p><p>I will try compiling the current CP2K master using the new make_cp2k.sh workflow, run the relevant regression tests, and then first reproduce your zero-field force-sum results on the same calcite frame. After that, I will check whether the same settings also resolve the sensitivity we observe in the finite-field response tensors.</p><p>Thank you also for pointing out the PRINT_LEVEL LOW workaround and the relevant GAPW_XC / GAPW_ACCURATE_XCINT settings. I will report back once I have tested these on our side.</p><div><br></div><p>Best wishes,<br>Kit</p></div></span></div><br><div class="gmail_quote"><div dir="auto" class="gmail_attr">On Wednesday, May 20, 2026 at 9:00:47 PM UTC+1 Thomas Kühne wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="line-break:after-white-space"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">Dear Kit,</div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">Thank you for the detailed report and for the input/output files. I was able to<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">reproduce the force-sum residual for the provided calcite frame:</div><div style="box-sizing:border-box;border:1px solid rgba(255,255,255,0.96);margin:0px;padding:0px;width:721.516px;min-width:0px;overflow:clip;border-radius:12.5px;background-color:rgba(26,28,31,0.08);color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-size:14px;font-variant-ligatures:normal;text-decoration-style:initial;text-decoration-color:initial"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex;border-top-left-radius:12.5px;border-top-right-radius:12.5px;font-size:13px;line-height:1.42857;color:rgba(26,28,31,0.494)"><div style="box-sizing:border-box;border:0px solid;margin:0px 0px 0px auto;padding:0px;display:flex"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span></div></div></div><div dir="ltr" style="box-sizing:border-box;border:0px solid;margin:0px;padding:8px;overflow:auto"><code style="box-sizing:border-box;border:0px solid;margin:0px;font-family:ui-monospace,SFMono-Regular,"SF Mono",Menlo,Consolas,"Liberation Mono",monospace;font-feature-settings:normal;font-size:1em;padding:0px!important;white-space:pre!important;border-radius:0px!important"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GPW baseline:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> Sum F = ( 3.48675295E-05, 1.12518883E-04, -5.39277596E-05 ) Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 1.29554803E-04 Ha/Bohr</span></span></code></div></div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">The behavior originate from the XC/grid treatment:</div><div style="box-sizing:border-box;border:1px solid rgba(255,255,255,0.96);margin:0px;padding:0px;width:721.516px;min-width:0px;overflow:clip;border-radius:12.5px;background-color:rgba(26,28,31,0.08);color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-size:14px;font-variant-ligatures:normal;text-decoration-style:initial;text-decoration-color:initial"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex;border-top-left-radius:12.5px;border-top-right-radius:12.5px;font-size:13px;line-height:1.42857;color:rgba(26,28,31,0.494)"><div style="box-sizing:border-box;border:0px solid;margin:0px 0px 0px auto;padding:0px;display:flex"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span></div></div></div><div dir="ltr" style="box-sizing:border-box;border:0px solid;margin:0px;padding:8px;overflow:auto"><code style="box-sizing:border-box;border:0px solid;margin:0px;font-family:ui-monospace,SFMono-Regular,"SF Mono",Menlo,Consolas,"Liberation Mono",monospace;font-feature-settings:normal;font-size:1em;padding:0px!important;white-space:pre!important;border-radius:0px!important"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GPW + USE_FINER_GRID T, FINE_GRID_FACTOR 2.0:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 2.26659720E-05 Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GPW + finer/smoothed XC grid:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 1.10078684E-06 Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GAPW_XC:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 4.20880179E-05 Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GAPW_XC + GAPW_ACCURATE_XCINT:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 1.68750841E-07 Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GAPW + GAPW_ACCURATE_XCINT + USE_FINER_GRID T, FINE_GRID_FACTOR 2.0:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 3.85198029E-08 Ha/Bohr</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">GAPW_XC + GAPW_ACCURATE_XCINT + USE_FINER_GRID T, FINE_GRID_FACTOR 2.0:</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> |Sum F| = 1.04105369E-08 Ha/Bohr</span></code></div></div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">This strongly suggests that the residual is dominated by an egg-box/grid-coupling<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">effect in the XC/grid integration rather than by pseudopotentials, basis sets, or<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">SCF noise.</div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">For this system, the best tested setting was:</div><div style="box-sizing:border-box;border:1px solid rgba(255,255,255,0.96);margin:0px;padding:0px;width:721.516px;min-width:0px;overflow:clip;border-radius:12.5px;background-color:rgba(26,28,31,0.08);color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-size:14px;font-variant-ligatures:normal;text-decoration-style:initial;text-decoration-color:initial"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex;border-top-left-radius:12.5px;border-top-right-radius:12.5px;font-size:13px;line-height:1.42857;color:rgba(26,28,31,0.494)"><div style="box-sizing:border-box;border:0px solid;margin:0px 0px 0px auto;padding:0px;display:flex"><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:flex"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;display:contents"></span></div></div></div><div dir="ltr" style="box-sizing:border-box;border:0px solid;margin:0px;padding:8px;overflow:auto"><code style="box-sizing:border-box;border:0px solid;margin:0px;font-family:ui-monospace,SFMono-Regular,"SF Mono",Menlo,Consolas,"Liberation Mono",monospace;font-feature-settings:normal;font-size:1em;padding:0px!important;white-space:pre!important;border-radius:0px!important"><span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">&QS</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> EPS_DEFAULT 1.0E-18</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> METHOD GAPW_XC</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> GAPW_ACCURATE_XCINT</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">&END QS</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"></span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">&XC</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> DENSITY_CUTOFF 1.0E-10</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> GRADIENT_CUTOFF 1.0E-10</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> TAU_CUTOFF 1.0E-10</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &XC_GRID</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> FINE_GRID_FACTOR 2.0</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> USE_FINER_GRID T</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &END XC_GRID</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &XC_FUNCTIONAL NO_SHORTCUT</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &GGA_C_PBE</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &END GGA_C_PBE</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &GGA_X_RPBE</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &END GGA_X_RPBE</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"> &END XC_FUNCTIONAL</span> <span style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">&END XC</span></code></div></div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><span>METHOD GAPW</span> with the same <span>GAPW_ACCURATE_XCINT</span> and fine XC grid also performs<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">very well, but in this test <span>GAPW_XC</span> was slightly better and should be the less<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">intrusive change relative to the original GPW calculation.</div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">One practical note: <span>GAPW_ACCURATE_XCINT</span> has been improved recently, so I would<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">recommend testing with a recent CP2K master or a release that includes those<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">changes. Also, for this calcite input current master hit a post-SCF output crash<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">with <span>PRINT_LEVEL MEDIUM</span>; using <span>PRINT_LEVEL LOW</span> while keeping explicit<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px"><span>&FORCES ON</span> avoided that and still printed the force block.</div><div style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px;font-size:14px;color:rgb(26,28,31);font-family:-apple-system,system-ui,"Segoe UI",sans-serif;font-variant-ligatures:normal;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">Best wishes,<br style="box-sizing:border-box;border:0px solid;margin:0px;padding:0px">Thomas</div><div></div></div></blockquote></div></blockquote></div><div 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