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

[Maxime Van den Bossche]

Dear Dorothea,

Thanks a lot for attending this issue (and for your efforts in
developing and implementing this method)! I hadn't noticed 
that the figure in your paper was also showing the presence 
of such short Pt-H distances.

So I'll conclude that this behaviour is a shortcoming of 
the Siepmann-Sprik potential (at least in the current,
canonical parametrization). As I'm not really comfortable
with such a discrepancy between the QM/MM and full-QM
results, I think I will be trying to amend the potential.

Could you further clarify why the cell size would be too 
small for IC-QM/MM calculations? Do you mean too small 
to be relevant to real interfaces? Then I agree, but this is 
just a test, and I would expect these short Pt-H distances
to also occur in larger cells (as you found yourself) --
if I replicate the structure in the X and Y directions, for
example, I assume I would be getting the same result.
Or do you mean something else?

Best,
Maxime


On Thursday, January 18, 2018 at 7:13:52 PM UTC, Dorothea Golze wrote:
>
> 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.... at gmail.com <javascript:>>:
>
>> 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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>
>
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