<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>Dear Haotian,</div><div><br></div><div>the key point is that "remove H but keep CHARGE 0" does not represent RCOO-.</div><div><br></div><div>Removing a neutral H atom while keeping the remaining system neutral produces</div><div>a neutral carboxyl radical or a different redox state. If the resulting</div><div>electron count is odd, it also requires UKS and an explicit multiplicity. Its</div><div>strong distortion is therefore not surprising.</div><div><br></div><div>Setting the total CHARGE to -1 gives the intended electron count for an</div><div>isolated slab+RCOO- system, but it also creates a net-charged periodically</div><div>repeated slab. Its electrostatic energy is not a vacuum-independent adsorption</div><div>energy unless a physical compensating charge, counterion or electrode model is</div><div>specified. The excess electron may also occupy a CsPbBr3 conduction state</div><div>instead of remaining localized on the ligand.</div><div><br></div><div>Changing SCF mixing settings cannot remove this physical ambiguity.</div><div><br></div><div>For the chemistry described in the question, I would first use a neutral,</div><div>atom- and charge-balanced dissociative adsorption reaction:</div><div><br></div><div> slab + RCOOH -> slab-RCOO + H_surface</div><div><br></div><div>and calculate</div><div><br></div><div> E_ads^diss =</div><div> E(slab-RCOO + H_surface)</div><div> - E(slab)</div><div> - E(RCOOH)</div><div><br></div><div>with CHARGE 0 for all three calculations.</div><div><br></div><div>Place the hydrogen on the chemically plausible surface site or sites and</div><div>compare the alternatives. If the experimental process is ligand exchange,</div><div>construct the corresponding balanced exchange reaction instead. If a</div><div>counterion, defect or electrolyte provides charge compensation experimentally,</div><div>it should be included explicitly.</div><div><br></div><div>For a genuinely two-dimensionally periodic neutral slab, CELL and POISSON</div><div>periodicity must agree:</div><div><br></div><div>&DFT</div><div> CHARGE 0</div><div> &POISSON</div><div> PERIODIC XY</div><div> POISSON_SOLVER ANALYTIC</div><div> &END POISSON</div><div>END DFT</div><div><br></div><div>&CELL</div><div> ...</div><div> PERIODIC XY</div><div>&END CELL</div><div><br></div><div>Alternatively, use three-dimensional periodicity with sufficient vacuum and a</div><div>dipole correction for a neutral asymmetric slab:</div><div><br></div><div>&DFT</div><div> CHARGE 0</div><div> SURFACE_DIPOLE_CORRECTION TRUE</div><div> SURF_DIP_DIR Z</div><div> &POISSON</div><div> PERIODIC XYZ</div><div> POISSON_SOLVER PERIODIC</div><div> &END POISSON</div><div>&END DFT</div><div><br></div><div>&CELL</div><div> ...</div><div> PERIODIC XYZ</div><div>&END CELL</div><div><br></div><div>The dipole correction does not make the energy of a net-charged slab</div><div>well-defined.</div><div><br></div><div>An expression such as</div><div><br></div><div> E(slab+RCOO-) - E(slab) - E(RCOO-)</div><div><br></div><div>is only meaningful after choosing and consistently applying the same physical</div><div>compensation model, cell, boundary conditions and finite-size treatment.</div><div><br></div><div>Newer CP2K versions provide PLANAR_COUNTER_CHARGE for explicit</div><div>counterelectrode models in three-dimensionally periodic charged-slab or GCE</div><div>calculations. This is a physical countercharge model, not a universal</div><div>numerical correction for arbitrary charged adsorption calculations.</div><div><br></div><div>Before comparing ligands, converge the lateral cell and coverage, slab</div><div>thickness, vacuum, cutoff and REL_CUTOFF, k-points and dispersion treatment.</div><div>Inspect the spin density, charge density and frontier orbitals to determine</div><div>where the extra electron or proton is located.</div><div><br></div><div>For adsorption from solution, solvent, counterions, proton chemical potential</div><div>or pH, and entropic contributions are additional parts of the adsorption free</div><div>energy.</div><div><br></div><div>I also checked the electrostatics numerically with CP2K. A neutral 2D test is</div><div>unchanged when the vacuum is increased, whereas the energy of an otherwise</div><div>equivalent charged 2D test changes by about 2.65 eV when the vacuum is</div><div>increased from 15 to 30 angstrom. This is why the very large charged-slab</div><div>binding energy is not reliable as currently defined.</div><div><br></div><div>Best regards,</div><div>Thomas</div><div><br><blockquote type="cite"><div>Am 07.07.2026 um 10:20 schrieb '舒夜' via cp2k <cp2k@googlegroups.com>:</div><br class="Apple-interchange-newline"><div><p>Dear all,</p><p>I am a beginner with CP2K. Recently, I have been trying to calculate the adsorption/binding energy of carboxylate ligands on a CsPbBr3 perovskite slab surface. Specifically, I would like to model the interaction between deprotonated carboxylate ligands, such as oleate (OA−) or benzoate (BA−), and surface Pb sites, and compare the relative binding strengths of different ligands.</p><p>The main issue I am facing is how to properly treat the charge of the deprotonated ligand in CP2K, and how to define a reliable binding energy.</p><p>So far, I have tested several models:</p><ol><li><p>I removed the H atom from the carboxyl group of the acid molecule and placed the deprotonated ligand near a surface Pb site of the CsPbBr3 slab, while setting the total charge of the whole system to 0.<br>The energy can converge in this case, but the ligand structure becomes significantly distorted after geometry optimization. Therefore, I am not sure whether this “H-removed but neutral total charge” model is physically meaningful.</p></li><li><p>I treated the deprotonated carboxylate ligand as an anion and set the total charge of the whole system to -1.<br>In this case, the SCF/energy convergence shows strong oscillations, and the calculated binding energy becomes abnormally large. The ligand may also become distorted during optimization.</p></li><li><p>I also tried setting the slab periodicity to XY and modifying the Poisson/electrostatic boundary condition settings, but I am still not sure what the correct setup should be for a slab + charged adsorbate system.</p></li></ol><p>I would like to ask the following questions:</p><ol><li><p>What is the recommended way to model the adsorption of carboxylate ligands such as OA−/BA− on a CsPbBr3 slab surface in CP2K?</p></li><li><p>For a deprotonated carboxylate ligand, should I directly calculate a charged slab + ligand anion system? Or is it more appropriate to construct an overall neutral dissociative adsorption model, for example:</p><p>neutral slab + neutral acid → slab–carboxylate + H_surf</p><p>where the H atom from the carboxylic acid is transferred to a surface Br site, so that the whole system remains at CHARGE 0?</p></li><li><p>If a charged system such as slab + OA− is used, how should CHARGE, PERIODIC, POISSON, and possible dipole/electrostatic corrections be set in CP2K? Would the adsorption energy obtained from such a charged slab system be reliable?</p></li><li><p>For the binding energy, which reference state is more appropriate?</p><p>E_ads = E(slab + OA−) - E(slab) - E(OA−)</p><p>or</p><p>E_ads^diss = E(slab–OA− + H_surf) - E(slab) - E(neutral OA)</p></li></ol><p>I would greatly appreciate any suggestions on the proper modeling strategy and binding energy definition.</p><p>Best regards,<br>Haotian Chen</p><div><br class="webkit-block-placeholder"></div>
-- <br>
You received this message because you are subscribed to the Google Groups "cp2k" group.<br>
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>.<br>
To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/964c77e8-3b8c-4748-a9b0-407e0e9a6accn%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/964c77e8-3b8c-4748-a9b0-407e0e9a6accn%40googlegroups.com</a>.<br>
</div></blockquote></div><br></body></html>
<p></p>
-- <br />
You received this message because you are subscribed to the Google Groups "cp2k" group.<br />
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>.<br />
To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/748B81EC-CCF9-455B-AEA9-DD50FA932EE5%40gmail.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/748B81EC-CCF9-455B-AEA9-DD50FA932EE5%40gmail.com</a>.<br />