[CP2K-user] [CP2K:18874] GEMC module does not keep stable liquid and vapor phases for water

[Vyacheslav Bryantsev]

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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