Water evaporation

GROMACS version:2020
GROMACS modification: Yes/No
Hello everyone,

I was trying to simulate the water evaporation process. I use the water model of TIP4P and the force field of OPLSAA. Steps of energy minimization, nvt, and npt looks good to me. I set the water at 298.15K at 1Bar.

After that, I used simulated annealing to heat the water from 298.15 to 523.15K in 50ns. However, by checking the potential energy, there is no significant fluctuation during the heating process so that I supposed no evaporation happened. Can some give me some suggestions to troubleshoot it? Thanks in advance!

Evaporation requires a phase change. i.e. your water molecules should show an increase in distance, and eventually density. I don’t think the methodology you applied will change any of these. Giving temperature basically increases the kinetic energy of your molecules drastically. But have you noticed any change in the box dimensions, or water density?

Hi Ali, thanks for your reply!

The density decreased during heating but did not reach the density of steam.

Are there any specific factors or methods that need to be applied in phase change simulation?

Well, I would personally do these kind of simulations on droplets, rather than a box of water. Something like what authors did here.

It might be that giving time to your simulations will eventually lead to very large box, with water density of a gaseous phase, but with a box filled with water it might take very long. A small droplet on the other hand in the middle of a simulation box might be rather easier to evaporate. It is only how I see it, and might not be the best solution. I’m pretty sure you can find many studies in the literature, just digging a bit. Here is another example I just found.

Thank you for the advice, I really appreciate it! I will try to decrease the number of water molecules.

As Ali mentioned, you want to simulate a phase change. This is hard for several reasons, and there is a lot of literature on how to simulate phase equilibrium (which is not exactly what you want, but is related).
Anyway, the main reason you are not seeing evaporation is that evaporation requires the nucleation of a bubble. And the formation of a bubble has an activation energy which renders it a rare event. In fact, it is easy to obtain superheated water even by heating a glass of water in a microwave, and boiling only starts when you add impurities (a tea bag, for example) that catalyze the nucleation of bubbles. Usually the walls of the container also catalyze the formation of bubbles (bubbles in champagne are formed on the glass). You simulation is of extremely pure water, without any boundaries and therefore, it does not boil. Or at least, it will take a lot of time to boil.
Starting with a droplet or a slab where you already have 2 phases should help. Still, water has a high surface tension so it can still take too much computational time.
Hope this helps.