# What are the specfic value of fundamental quantities of mass-m, length-σ,energy-ε

Hello. I got a peoblem when I want to convert physical quantities into unitless quantities.

My model is 10 discrete particles and I used the discrete element method. So my simulation is quite like the granular example, which is in lammps/in.pour.drum at develop · lammps/lammps · GitHub. Also I used command “units lj”, but I do not know how to convert physical quantities into unitless quantities. For example, mannual said I should divide the real value M by fundamental value m, and I would get a unitless m*. However, I do not know the specfic value of m.

Moreover, I also do not know how to convert the time τ. It should be determined by energy ε according to the manual. But since I considerd friction, there is no exactly expression for energy.

What do these 10 discrete particles represent in reality? What system are you trying to model?

Hello,

This is not the first time that the use of Lennard-Jones units brings confusion here on this forum. So my first question would be, do you need to use lj units? This system of units is great when you want to simulate a very generic system, but if your goal is to model an existing material/gas, then don’t use it.

Note that, regardless of the system of unit you choose, you must know the mass m of your atoms. If you don’t, I doubt that people here will find it for you, particularly since you did not give much details about your system (cf the question of @Raghurame).

Best,
Simon

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Thanks for your reply. I tried to reappear an article, and that paper said that the particle’s mass is 1 and the ridus r is 0.5. So I also can not tell the exact value. And the paper said length is measured in 2r and mass is measure in m. So how should I adjust my units in Lammps to match the units in the paper.

Hi @Wilder,
As stated by @simongravelle, this topic has been tackled a lot in the forum and in the archives of the mailing list. You might find answers to your problem by typing `lj units` in the search bar (we recently gave yet another detailed answer here).

I suggest you read the units command manual page to understand how units are related to one another in lj units and how to convert them back to real units. You only need to choose a mass, a length and an energy scales. The time unit has the dimension of… time so it can be computed both using sqrt(mass*length^2/energy) or sqrt(mass*length*force constant) depending on which one you choose to fix in your parameters.

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I don’t understand what you are asking here. The text you quoted already uses reduced units. So how do you need to still convert them?