[CP2K-user] [CP2K:19151] Re: GEMC module does not keep stable liquid and vapor phases for water

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

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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