[CP2K-user] [CP2K:11464] Re: Water-bilayer/Metal interface - Ab intio MD

Marcella Iannuzzi marci... at gmail.com
Thu Mar 21 22:26:04 UTC 2019

Dear Anna,

no I think that the potential you apply is wrong, indeed the forces in z on 
the waters' atoms are insane.
Is you slab metallic? If yes, I do not believe that the DFT settings of 
your input give reasonable results.
In this case the occupation of the molecular orbitals around the Fermi 
energy should be smeared. 
Indeed from the output you sent it turns out that the SCF convergence is 
very slow and shaky. 
Even if this is not the case, a PW cutoff of 200 Ry is very small, and I 
would really advise to employ some vdw correction scheme. 
Moreover, you should check the electronic structure (e.g. DOS).

>From the results of the MD simulations with the cell filled with water, you 
should observe bulk liquid water behaviour in the middle.
This can be verified from known properties like the density, the radial 
distribution function, etc. 
You should also be able to estimate a sort of interaction energy between 
the water slab and the surface. 
If you expect the double layer alone to be stable on the surface, there 
should be some kind of adsorption. 
Hence it is sensible to determine the optimal adsorption geometry and the 
adsorption energy for individual molecules. 
If the adsorption strength is too weak, it makes sense that at room 
temperature the molecules fly away. 
Maybe joust adding vdw terms solves the problem. 
Anyway, I would not try to force it with an arbitrary external potential


On Thursday, March 21, 2019 at 8:43:25 PM UTC+1, Anna Anic wrote:
> Dear Marcella,
> Thank you very much for the quick response. 
> To answer your questions:
> 1. Before I did the MD part I performed energy convergence test for the 
> slab thickness and size, then a geometry optimization for the bulk and slab 
> with and without adsorbate with a 20 Angstrom thick vacuum level
> But I just realized that the coordinates in the input file are not from 
> the optimized structure. But I think that the water bilayer still should 
> not behave like this
> 2. All structures converged during these calculations and surface free 
> energies (SFE) are similar to reference values from other works
> 3. I did not calculate the binding energy for water molecules. Is this 
> relevant if the SFE values seem to be oke?
> 4. The layers of the water molecules should represent the water structure 
> in the Double Layer of an electrode/electrolyte interface. I took the 
> geometry of the bilayer from a paper, that claims that the water at the 
> interface is ice-like. I want to check if it stays that way (probably not) 
> and sample as many water orientations (as this changes the work function) 
> as possible.
> 5. Concerning the MD part I did not anneal the structure slowly. But I did 
> the same calculation with a fully solvated system, so the whole vacuum 
> region was filled with water, and the movement of the water molecules 
> seemed reasonable. 
> What temperature steps for the anneling would you suggest ?
> And do you maybe have some example input block for that?
> In general, do you think that the external potential with the parabolic 
> function is oke to keep the water molecules within a certain region from 
> the surface?
> Greetings,
> Anna
> Am Do., 21. März 2019 um 17:00 Uhr schrieb Marcella Iannuzzi <
> marc... at gmail.com <javascript:>>:
>> Dear Anna, 
>> which type of calculations did you carry out in preparation of the MD run?
>> Is your DFT model reproducing the correct lattice structure?
>> Did you optimise the slab and check that the electronic structure of the 
>> bulk and at the surface is what expected from reference data?
>> I do not know this specific system, but the parameters in the DFT section 
>> seem to be not very well tuned. Is the SCF converging? Do you get 
>> reasonable forces?
>> Did you model the adsorption of individual water molecules to evaluate 
>> the binding energy and compare to references? 
>> Should the layer of molecules be ice-like or do you want to simulate 
>> liquid water?
>> If the coordinates are obtained from some structural data or geometry 
>> optimisation, did you anneal slowly from 0K to the final temperature?
>> Why are you running NPE? Did you already thermalised at constant volume?
>> Kind regards
>> Marcella
>> On Thursday, March 21, 2019 at 3:55:19 PM UTC+1, Anna Anic wrote:
>>> Hey everybody,
>>> I am trying to do Ab-initio MD for a metal-water interface. I am using a 
>>> symmetric slab and positioned a bilayer of water at both surface sites. 
>>> Now, if I run the simulation my water layer just decomposes into the 
>>> vacuum region. That is why I would like to implement an vacuum potential 
>>> barrier at some point in the z-direction to prevent my water from just 
>>> "flying away".
>>> I tried using the &EXTERNAL_POTENTIAL keyword and defined a harmonic 
>>> function as a barrier.
>>> But when I run the simulation the hydrogen atoms from the H2O-bilayer  
>>> are "blown away".
>>> The question is, what did I do wrong? Is the idea legit? Does anyone 
>>> have a proposal how to fix the problem or how to do it differentey.
>>> Greetings,
>>> Anna
>>> (I attached my input file,geometry and trajectory )
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