[CP2K:9886] Very short metal-H distances from IC-QM/MM

[Dorothea Golze]

Hi Maxime,

there is most likely nothing wrong with the force field implementation etc.
I ran quite a few simulations with the Siepmann-Sprik forcefield + image
charge setup and did not encounter any problems so far.
Some explanations:
The Pt-O distance is approximately 2.4 Angstrom with the Siepmann-Sprik
force field (see https://doi.org/10.1063/1.469429 Tab. 1). In most cases,
the O-H bonds are roughly parallel to the surface or point away from it.
However, in some cases the H atoms point towards the surface, and in this
case the Pt-H bond is only 1.5 Angstrom. This is demonstrated in Figure 11
(d) in https://doi.org/10.1021/ct400698y .The onset of the red curve is
already at 1.5 Angstrom (but the peak is at 2.4 Angstrom, so most Pt-H
distances are 2.4 Angstrom).
Another important point, your "super-cell" is only 3x4 gold atoms. That's
way too small. Might be that you also see some more funny effects due to
this.

Your points above

>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).

No, it is also correct that you see the same results with different IC
Gaussian widths (that's good) . Keep the default.

>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?
Of course you can always reparametrize if you wish, however, I think there
is not actually a problem, see above.

>c) ... this is related to the implementation of the
       Siepmann-Sprik potential and/or image charges
       in CP2K?
>d) ... something else?
see above


Best regards,
Dorothea

2018-01-18 16:30 GMT+02:00 Maxime Van den Bossche <
maxime.cp.v... at gmail.com>:

> 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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