# In eDPD pair_coeff what are the cut_offs refer to?

Dear All,

Lookind at the Pair_coeff for Pair_style edpd documentation

https://docs.lammps.org/pair_mesodpd.html

I cannot understand the meaning of few coeffs. What are power_f and power_T?
what about cut_off? is this cut_off the same as what we define in ''comm_modify vel yes cutoff 13" or different?
what is cutoff_T?

other parameters I can guess from the equations. Is A suppose to be the repusion parameter known as (a_ij in most DPD papers)?

Thanks a lot

You have to properly study the publication(s) describing the models. This is standard research protocol. The references are given at the bottom of the documentation page. A software manual is like a ownerâ€™s manual of a car: it only tells you where the knobs are and what settings are available, it will not teach you how to drive it.

Never guess. That is the first step down the road to making embarrassing mistakes and possible doom. Do not use a model or a feature until you have understood what it does and how it works. There are many ways where you can get simulations with results that look reasonable, but are still entirely bogus due to bad settings and parameters.

In the publications we have the same formulas, in the documentation it talks about the knobs A and Gamma which they appear in the formulas given in the LAMMPS documentation as well. but the knob Power_f and power_T cannot be seen in the explanations. papers do not talk in LAMMPS language. I agree that to understand the physics we need to go to papers. I am not asking that the documentation teach how to drive. It is not even clear which knob refer to which element in the formula that is documented. I am not asking for physical meaning, Just asking where are they in the formulas so I can figure out the physics? For example I can find A and Gamma in the formulas which is great ! but itâ€™s not clear where the other coefficients belong.

Thank you so much Dr.Kohlmeyer for always responding so fast .You helped me a lot in learning, but I respectfully disagree with you that I could understand what the author means by power_T if they are not in the formulas.

I cannot give you that explanation. This is not my area of research. What you can try is to contact the author of the pair style / package that these styles belong to or the authors of the publications.
I am quite confident that the information you are looking for is somewhere in the published literature, it is not always as straightforward to find it. You most certainly need to spend more time on reading it completely not just checking with some expressions.

The name of the contributing author is given in the â€śPackage detailsâ€ť manual page and some contact info should also be in the source code. Perhaps you can convince that person to submit an update to the documentation and discuss the specific problems with him (or - after you have understood the details with help from the contributing author - you can even provide a modification to the manual yourself. You will be most certainly in the best situation to change the docs so that a person not familiar with the model can understand it better. Developers often struggle to see what matters because to them everything is â€śobviousâ€ť).

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Sure, thank you. I will update what I understand here. Thanks again.