[CP2K-user] [CP2K:19157] Re: GEMC module does not keep stable liquid and vapor phases for water

[Vyacheslav Bryantsev]

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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