<div dir="ltr">Dear all, I've been interested in applying the IC-QM/MM approach implemented in CP2K to investigate certain metal-water interfaces. I would like to describe the water with DFT, the metal using some forcefield, and the metal-water interactions via the Siepmann-Sprik potential plus image charge electrostatics. During my initial testing of H2O layers on a Pt(111)-c(3x4) substrate, I found some of the structures (with the bottom H2O molecules adsorbed H-down) to display overly short Pt-H bond lengths (1.6 Å). I've attached the coordinates of such a structure, and the input with which this (relaxed) structure was obtained. For simplicity / familiarity, the input file is essentially identical to that of the online Pt(111)-H2O how-to example. The output (with CP2K version 6.0, commit 8b033c3) is also attached. As can be seen from the 'run-r-1.out' file, the Pt atoms underneath the down-pointing H atoms acquire a negative image charge of around -0.07 a.u., which still seems to be reasonable. When running a full DFT (PBE-D3) optimization of the same structure, the Pt-H bond lengths expand to more believable values of around 2.15 Å. So, I'm wondering what is going on, because there seem to be different possibilities. Do you think ... a) ... something is wrong in my input? I also got the same results with narrower Gaussians for the image charge distributions (5 Å^-2 instead of 3.5 Å^-2). b) ... this is related to the Siepmann-Sprik potential parameters? I couldn't find previous works, though, (such as this forum, the original paper, or the implementation paper) reporting such behaviour. But if this is the cause, I could just add a short- ranged repulsive potential to the Pt-H interaction or maybe modify the Phi function in Eq 3 of the original paper. Could it be that one really needs to reparametrize the potential because of the differences between DFT-water and SPC/E-water? c) ... this is related to the implementation of the Siepmann-Sprik potential and/or image charges in CP2K? d) ... something else? The online how-to: https://www.cp2k.org/howto:ic-qmmm The original Siepmann-Sprik paper: https://doi.org/10.1063/1.469429 The IC-QM/MM implementation paper: https://doi.org/10.1021/ct400698y Best, Maxime </div>