<div>Hi Nick, <br /></div><div>I cannot really provide input on the QM/MM part as I have no expertise in this. For your SCF convergence issues, you might want to look into disabling safe DIIS, which uses steepest descent if the DIIS algorithm does not decrease the energy (https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/SCF/OT.html#CP2K_INPUT.FORCE_EVAL.DFT.SCF.OT.SAFE_DIIS). I've found it sometimes causes issues.</div><div>On a more general note, for light atoms like those you are using, you might get similar results and better convergence using the Gaussian-Plane-Wave method (GPW) instead of GAPW, and using the GTH effective core potentials optimized for PBE0 and associated TZVP or TZV2P MOLOPT basis sets (which are available here https://github.com/juerghutter/GTH/blob/master/PBE0/POTENTIAL for the ECPs and here for the basis sets https://github.com/juerghutter/BASIS/blob/master/MOLOPT/PBE0/MOLOPT_PBE0). This would dramatically reduce the number of electrons.</div><div>Take care, <br /></div><div>Q.</div><div class="gmail_quote"><div dir="auto" class="gmail_attr">Le lundi 10 mars 2025 à 03:31:39 UTC+1, Nicholas Laws a écrit :<br/></div><blockquote class="gmail_quote" style="margin: 0 0 0 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">Hi Quentin,</span></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><br></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">Thank you for your thoughtful suggestions. I have implemented your recommendations, all of which have certainly improved the computational efficiency. However, I am still encountering persistent SCF convergence failures at the moment of collision with the external potential, regardless of whether diffuse functions are included in the basis set (I tested aug-cc-pVTZ, cc-pVTZ, def2-TZVPPD, and def2-TZVPP).</span></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><br></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">To circumvent the challenges posed by the external harmonic potential, I created a QM/MM workflow that models the Au surface classically. Unfortunately, similar SCF convergence issues arise when the EMI-BF4 neutral collides with the Au surface in the QM/MM framework. I was curious if there are any gaps in my current QM/MM framework that may cause this lack of SCF convergence? Additionally, I was curious if you or the community could recommend accurate sources or standard references for Lennard-Jones (LJ) pair potentials between the relevant QM atoms (N, C, H, B, and F) and Au atoms? Currently, I am using Lorentz-Berthelot combing rules from the </span><a href="https://openkim.org/id/MO_959249795837_003" target="_blank" rel="nofollow" data-saferedirecturl="https://www.google.com/url?hl=fr&q=https://openkim.org/id/MO_959249795837_003&source=gmail&ust=1741679886406000&usg=AOvVaw1m1_Tng6A3pP-Qe5_98uRs"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;background-color:transparent;font-style:italic;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">Efficient 'universal' shifted Lennard-Jones model for all KIM API supported species developed by Elliott and Akerson (2015) v003</span></a><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-style:italic;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">.</span></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><br></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">I have attached the relevant input file for your review in .zip format. Thank you again for your valuable advice, and I look forward to any insights you or others in the CP2K community might have.</span></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><br></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">All my best,</span></p><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:10.5pt;font-family:Roboto,sans-serif;color:rgb(0,0,0);background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;font-variant-alternates:normal;vertical-align:baseline">Nick</span></p></blockquote></div>

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