[CP2K-user] [CP2K:19167] Re: GEMC module does not keep stable liquid and vapor phases for water

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

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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