[CP2K-user] [CP2K:19156] Re: GEMC module does not keep stable liquid and vapor phases for water

['sing...@umn.edu' via cp2k]

Moreover, I would like to add that people experienced in FPMC using CP2K 
have told me that it does not matter much if you do not have a very good 
force field for your molecules. The biasing potential (ff) is more or less 
used to avoid overlaps during pre-sampling. You could estimate the epsilon, 
sigma, and charges using established FFs and the bond and angle constants 
can be estimated by fitting a parabolic function to the bond length and 
angle energy scan (you could do that in Gaussian). 

But, as I do not know about your system, I do not know if it would still be 
applicable for the molecules you want to simulate.

Best,
Ramanish

On Friday, July 28, 2023 at 9:49:56 PM UTC-5 sing... at umn.edu wrote:

> 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?
>
> Best,
> Ramanish
>
> 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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