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

Anna Anic aan... at gmail.com
Thu Mar 21 19:43:11 UTC 2019

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?


Am Do., 21. März 2019 um 17:00 Uhr schrieb Marcella Iannuzzi <
marci... at gmail.com>:

> 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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