What the most accurate potential to use for the simulation of gas diffusion in PE?

Dear LAMMPS users,

I’m a new beginner using LAMMPS for molecular dynamic simulation. I am currently working on modeling the sorption and diffusion between polyethylene and methane using LAMMPS.

I obtained a polyethylene system from this source: Atomistic Deformation of Amorphous Polyethylene - EVOCD
I used the Avogadro software to generate the methane molecules.

Now, I need to model their interaction and observe how the pressurized gas penetrates through the polymer. In order to do this, I need to determine the appropriate interaction potential between the polyethylene and the gas. However, I am unsure which one to use. Perhaps someone could provide some guidance?

Thank you in advance.

Kind regards,
Bo

This is a rather challenging subject and not a good choice for a beginner.

This uses the Dreiding force field. This is a force field that is primarily optimized for representing molecular structures, but not at all for non-bonded interactions. For the specific application of deforming PE the former seems to be the interaction dominating the results, but for your subject that would not be the case. You would be better off looking for a potential that is optimized for non-bonded interactions between PE and methane or for hydrocarbons in general. OPLS sounds like a more applicable choice (which exists in both united atom (UA) and all atom (AA) representations). The choice between UA and AA may have an impact on your results, too.

The major problem for your topic is that the gas diffusion through a dense medium is an activated process and thus very slow on the time scales available to MD simulations by themselves. You will have to simulate a very large system for a very long time to get sufficient statistical data and equilbration will consume a lot of that time. That is rather wasteful. A better strategy would be to use a hybrid MD/MC approach to accelerate the diffusion by inserting MC moves into the MD simulation.

Since this is not a new subject, the best approach is to fist do an exhaustive study of the available published literature on gas diffusion through solids or polymers. There should be plenty. Do not rely just on some tutorial.

Thank you very much for your quick response.

I would like to clarify if I have understood your point correctly: the Dreiding force field is used to describe a single structure, but it is not suitable for describing the interaction between two different structures, is that correct?

Don’t take my word for it. Instead, please study the paper describing the Dreiding force field and what kind of tests were performed to validate it and draw your own conclusion. Blindly following what is posted on the internet can lead to gross errors.

If you want to get answers that sound reasonable but are just assemblies from related materials on the internet without careful scientific review, you can save some time and ask ChatGPT. :wink:

Thank you very much for your help!
I decided to clarify because English is not my native language, and I just wanted to understand if I understood you correctly. :slightly_smiling_face:

English is not my native language either, so even if you understand me correctly, how can you be certain that I understood you or understood the force field?
The only way to be certain is to go to the original source!