Hello everyone, I am currently learning how to perform slow-growth simulations in CP2K to obtain free-energy profiles. I followed the exercise “NaCl Free Energy Dissociation” from the CP2K website (<a href="https://www.cp2k.org/exercises:2015_ethz_mmm:nacl_free_energy">exercises:2015_ethz_mmm:nacl_free_energy</a>), which uses PMF to compute the free-energy surface. Based on that, I tried to reproduce a similar NaCl dissociation process using the following constraint setup: <div><div><div><div></div></div></div><div dir="ltr">&CONSTRAINT &COLLECTIVE COLVAR 1 INTERMOLECULAR TARGET [angstrom] 2.4 TARGET_GROWTH [angstrom*fs^-1] 1E-4 TARGET_LIMIT [angstrom] 6.0 &END COLLECTIVE &LAGRANGE_MULTIPLIERS COMMON_ITERATION_LEVELS 1 &END &END CONSTRAINT</div></div> This means that the Na–Cl distance is slowly increased from 2.4 Å to 6.0 Å at a rate of 1E–4 Å/fs. The simulation itself runs smoothly, but the results are somewhat puzzling. As shown in the figure below, as the distance increases, the recorded constraint force gradually approaches a small negative value close to zero, instead of approaching zero from either side. Because of this, when I integrate the force to obtain the free-energy profile, the resulting free energy continues to increase in the long-distance region rather than converging to a plateau — which does not seem physically reasonable for a dissociating ion pair.<img alt="shake_smooth1.png" width="535px" height="334px" src="cid:3ff0dcf1-92d9-44b4-a937-c3cfa56df458" /> <img alt="shake_integral.png" width="535px" height="334px" src="cid:5584736a-9beb-44ff-a25c-9250cc6410e0" /> For reference, my system contains 1 Na⁺, 1 Cl⁻, and 63 H₂O molecules, exactly the same as in the CP2K exercise mentioned above. My full input files are attached below. I would greatly appreciate any suggestions or insights into why the constraint force at large separations does not decay to zero, and how to correctly obtain a converged free-energy profile. Thank you very much! -- 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/27df279d-fcd6-427b-8405-8c1bb9ee4a77n%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/27df279d-fcd6-427b-8405-8c1bb9ee4a77n%40googlegroups.com</a>.