[CP2K-user] [CP2K:18889] GEMC module does not keep stable liquid and vapor phases for water

[Jürg Hutter]

Hi

I don't think that there are still active developers for the GEMC modules. We try to keep the
known workflows running, but you are on your own if you depart from the tested inputs.
Of course we would be happy if somebody would take over further development of these routines.

regards
JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Vyacheslav Bryantsev <vyacheslav.bryantsev at gmail.com>
Sent: Wednesday, May 31, 2023 1:54 PM
To: cp2k
Subject: [CP2K:18874] GEMC module does not keep stable liquid and vapor phases for water

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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