[CP2K-user] [CP2K:19154] Re: GEMC module does not keep stable liquid and vapor phases for water

[Vyacheslav Bryantsev]

Hi Ramanish,

Thank you for your comments and a suggestion. 
I am looking for a way to run GEMC simulations in the case when we do not 
have a good force field.
After I turned off the  LBIAS and made the DFT method exactly the same in 
two boxes, I still experience the same problem of getting water distributed 
about equally between the two phases. 
The two boxes input files along with the output *mc_molecules* file showing 
a drift of water molecules from one box to the other are attached. 
Again, if the force field is used in the second box, as in the provided 
example, https://www.cp2k.org/howto:gemc,  there is no issue. 

Therefore, the problem of doing all the MC moves with DFT still persists. 

Thank you,
Slava



On Friday, July 28, 2023 at 1:36:05 PM UTC-4 sing... at umn.edu wrote:

> Hi Vyacheslav,
>
> Could you please elaborate why you want to use DFT as biasing potential 
> instead of a force field (FF) (for e.g. TIP3P)?
> My understanding of the biasing potential is that a FF can be used to 
> conduct some computationally cheap steps between two expensive DFT energy 
> evaluations. If LBIAS is on, there will be NMOVES number of steps conducted 
> using the FF you define in the biasing potential. 
>
>  [image: Capture.PNG]
> At the end of FF pre-sampling NMOVES, there is a comparison between the 
> change in DFT energy and FF energy b/w the initial and final stages to 
> determine if the final state would be accepted or not. 
>
> So, if you want to use DFT as your potential for pre-sampling, then it 
> doesn't make sense to have LBIAS on. You can set LBIAS to FALSE, and do all 
> the MC moves based on the DFT potential you define in box1.inp and box2.inp.
> Let me know if this makes sense.
>
> Best,
> Ramanish
> On Wednesday, May 31, 2023 at 6:54:19 AM UTC-5 Vyacheslav Bryantsev wrote:
>
>> Dear CP2K Developers,
>>
>> I am looking into using the Gibbs Ensemble Monte Carlo (GEMC) methos in 
>> CP2K to compute the liquid-vapor phase diagram using a fully DFT-based 
>> method.
>>
>> However, I have problems stabilizing liquid and vapor phases when doing 
>> simulations fully ab intio, as described below.
>>
>> By taking input files for water from
>>
>> https://www.cp2k.org/howto:gemc 
>>
>> and minimally changing them, I was able to run the GEMC model and get 
>> sensible results for water. This input utilizes the BLYP functional and an 
>> empirical model for pre-sampling, employing the FIST methods as specified 
>> in the bias_template.inp file. The input files and the output mc_molecules 
>> file are attached. The latter file  shows how many molecules are present 
>> during the GEMC run in the liquid phase (between 64 and 62 water molecules) 
>> and the gas phase (between 0 and 2 water molecules).
>>
>> The problem starts where I substitute the empirical potential for water 
>> with the actual BLYP method in the bias_template.inp file (I renamed that 
>> file to be bias_template-DFT.inp). Please note that I keep all other 
>> settings the same as before, but only substituting the empirical water 
>> model with the DFT model.  Now, the results of simulations have no sense. 
>> The water molecules from the liquid box move to another gas phase box until 
>> two phases have roughly the same number of water molecules. The  
>> mc_molecules-DFT shows this problem. Using fully DFT calculations is 
>> supposed to give very similar or slightly improved results, but instead it 
>> produces very different and unexpected results. It looks like that 
>> replacing the FIST model with the DFT model causes some changes in the 
>> program that treats swapping molecules between the two boxes differently 
>> (almost like allowing them to swap only one way). 
>>
>> At this stage, it is crucial to consult CP2K developers to identify the 
>> factors that contribute to a significant change in the software's behavior 
>> when transitioning from the utilization of the empirical potential in 
>> bias_template.inp to using DFT. 
>>
>>
>> Thank you,
>>
>> Slava 
>>
>> Vyacheslav Bryantsev
>>
>> Chemical Separations Group
>>
>> Oak Ridge National Laboratory
>>
>

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