Very short metal-H distances from IC-QM/MM

[Maxime Van den Bossche]

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
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