[CP2K-user] [CP2K:19155] Re: GEMC module does not keep stable liquid and vapor phases for water

'sing...@umn.edu' via cp2k cp2k at googlegroups.com
Sat Jul 29 02:49:56 UTC 2023

Hi Slava, 

Thanks for explaining the issue. I think I can try to run for this case and 
see if I face the same issue. Can you please tell me after how many steps 
you see the densities of the two boxes becoming nearly equal?


On Friday, July 28, 2023 at 7:37:39 PM UTC-5 Vyacheslav Bryantsev wrote:

> 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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