[CP2K-user] [CP2K:19169] Re: GEMC module does not keep stable liquid and vapor phases for water

Vyacheslav Bryantsev vyacheslav.bryantsev at gmail.com
Thu Aug 3 02:10:26 UTC 2023

Thank you, Ramanish, for looking into this.
I tried many options with DFT, but nothing worked for me.
However, when I do use the biasing potential as in the provided example (
https://www.cp2k.org/howto:gemc), then everything seems to be working fine. 
A complete input file with a biasing potential is attached.
I hope this could help to track down the problem.

Thank you,

On Wednesday, August 2, 2023 at 3:30:47 PM UTC-4 sing... at umn.edu wrote:

> Hi Slava, 
> I ran your files with the BLYP functional and PBE functional. You 
> prediction was correct, I did see the density of the two phases becoming 
> the same (see attached files). Dashed line denotes the density of box1 and 
> the solid line denotes the density of box2.
> [image: Capture.PNG]
> [image: Capture.PNG]
> Previously I was suspecting the the critical temperature of BLYP-water is 
> lower than 650 K and that's why we see same density for the two boxes at 
> 473 K, however, to my surprise, we observe the same result for PBE-water, 
> which is supposed to have a Tc much higher than 650 K. I will need to do 
> some more tests to check this particular behavior. I will run the VLE 
> simulation with some other combinations of basis_set/cutoff/functional. 
> Best,
> Ramanish
> On Monday, July 31, 2023 at 5:10:24 PM UTC-5 sing... at umn.edu wrote:
>> Thanks for providing your files Slava.  I will run for this case.
>> I am not sure if FIST can handle polarizable FF.  I guess you can still 
>> estimate and come up with a non- polarizable FF to be used in FIST 
>> (pre-sampling).
>> I will let you know what I get from running your water files.
>> On Saturday, July 29, 2023 at 1:02:50 PM UTC-5 Vyacheslav Bryantsev wrote:
>>> Hi Ramanish,
>>> Please find a complete input file attached to this email.
>>> It does take some time to see the problem. After a few hours run on 3 
>>> intel nodes (36 cores) 12 water molecules moved from one box (liquid) to 
>>> the other (gas).
>>> This would continue until about an equal density in each box. 
>>> I get your point about having a decent force field for doing 
>>> pre-sampling. 
>>> Would it be possible to use a polarizable force filed (PIM model) in the 
>>> FIST module of CP2K? This is for modeling  liquid AlCl3 consisting of pure 
>>> Al2Cl6 dimers. The PIM force field is available for Al2Cl6.
>>> Thank you,
>>> Slava
>>> On Friday, July 28, 2023 at 10:55:32 PM UTC-4 sing... at umn.edu wrote:
>>>> 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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