# Large crystal generation(NaCl) from unit cell

I’m working on a project titled “Dissolution of NaCl in water”. I have developed a NaCl unit cell using LAMMPS, but I need to generate large crystals (more number of crystals) from the unit cell. Can someone explain me on how to proceed?

Two questions:

1. How large should your individual salt granules be?
2. What can you learn from having multiple granules, that you cannot learn from studying a single one?

I need to have sizes of 500 micrometer particle and determine the size and shape during dissolution into water at different volumes. This involves salt as source of ions and water to accumulate the salt ions.
Would equilibrium state stops the dissolution or would it be dynamically equilibrium leading to an existence of an interfacial surface? If the latter case, then I need to determine the characteristics of the interface.

Have you made an estimate about how many atoms would be needed to have a single 500 micron size object? For multiple of those and sufficient dilution you need this size many times over.
Then you need to explain what kind of supercomputer access you have and what kind of experience in running such extremely large calculations.

My interest is to identify any physical interface around a dissolving particle. Size can be reduced to even few micrometers. I have only a laptop with 8GB ram and 2.70GHz processor

So let’s just do a little back-of-the-envelope estimate. The NaCl conventional unit cell is 0.56x0.56x0.56nm^3 and has 8 atoms. So to reach a micrometer, you would need replicate this 2000x2000x2000 time, which are 64 billion atoms. And that is the NaCl crystal only.

But there are other things to consider:

• how large a water layer do you need around this? so you may be looking at systems 1000 times larger (which is only a 1:10 ratio in each direction)
• LAMMPS has limitations as to how many atoms per MPI rank you can simulate and this would be beyond that
• you will need to do long-range electrostatics which parallelized only O(N log(N)) once the 3d-Fourier transform becomes a relevant term, i.e. when you grow the number of CPUs you use
• what time scale are the processes on that you want to study. there are same as the size limitations, you also have time limitations. while you may be able to run a tiny system for miliseconds (your time step is of the order of a femtosecond), this will be near impossible for a huge system
• what kind of potential can actually model the process accurately? you need something that is good to represent the solid, the interface and the interaction with water at the interface and the solvated ions. not an easy feat.
• what impact does structuring of the surface have? how much do you know whether structural defects matter? or whether dissolving ions from an ideal surface is an activated process?

Overall, I don’t think this is something that is as easily doable with classical MD as you think. Have you done any research of the published literature about what kind of research people have done to study dissolution of salts in water? or any kind of solid into a solvent? For all I know, this is a “hard” problem and most certainly not accessible by a brute force “simulate and a see what happens” approach. From my limited experience, I would expect a method like kinetic monte carlo be more suitable, but then again, how would one parameterize that?

I see, thank you for the information that you have provided. My interest is to verify if there is any physical interface do exist.

Would it be possible to take wall three molecule layers and 50 molecular layers of water? The wall height could be around 1000 molecules. Would this be possible to study ionic-distribution in water layer and identify presence of an interface with steep gradients?

I am not the published literature, I am not your adviser, and I am not interested in this kind of research. So you will have to consult available sources that have this kind of information (as I already suggested) and talk to people that have an interest in this area of research.

Besides, if the only computational resource available is you laptop, then you will have to be extremely smart in designing meaningful simulations where you can extract useful information from without having to wait a few years for the results (and only to find out that you made a mistake in your input and have to repeat the calculations). This will be the case for about any kind of MD simulation that you would want to do.

Yeah I can understand Sir. Thanks for your time.