# How to get mainly the long range interaction of LJ potential

Dear All,

I knew that in lammps, the WCA potential just keep the repulsive part of LJ potential (cut off 2^(1/6)).

How can I just keep mainly the attractive part of LJ potential in lammps?

Best Wishes!

Dear All,

I knew that in lammps, the WCA potential just keep the repulsive part of
LJ potential (cut off 2^(1/6)).

How can I just keep mainly the attractive part of LJ potential in lammps?

without some more context, this questions does not make a lot of sense.​

you can include as long a range as you want by increasing the cutoff and
even go to infinity, when using the dispersion pppm.
however, you *cannot* run an LJ potential *without* the short range
interactions. it simply won't be an LJ potential anymore.

axel.

Hi, Alex,

Thanks for the information.

I am studying the paper : http://scitation.aip.org/content/aip/journal/jap/93/10/10.1063/1.1540169

The whole system as below:

I plot the equation 2 (blue) and 2+3 (red) , also the LJ 126(green)

and noticed that equation 2+3 looks like the attractive part of LJ

So I want to use just the attractive part of LJ to represents 2+3 since I don’t know how to implement or modify
those two equations in LAMMPS.

Best Wishes!

Hi, Alex,

Thanks for the information.

I am studying the paper : http://scitation.aip.org/content/aip/journal/jap/93/10/10.1063/1.1540169

The whole system as below:

I plot the equation 2 (blue) and 2+3 (red) , also the LJ 126(green)

You can use any pairwise additive potential in lammps via pair_style table.

That one requires a bit of tinkering to get things to work well and smoothly. Thus it is highly recommended to first practice to tabulate a potential that exists in its analytical form to test tabulation parameters for accuracy and energy conservation.

It seems Yanqing need article wall interaction.

I may be wrong, but, I can’t find a tabulated interaction for particle wall in lammps. I am afraid some coding is needed.

Best,
Yi

It seems Yanqing need article wall interaction.

​yes. i couldn't see the pictures on the cell phone and was assuming (as
was the question suggesting) that this was about a pairwise potential.
and this model doesn't need just one wall​ potential but *two*.

also, the LJ wall is a 9-3 wall and has only a vague resemblance of a 12-6
LJ.

I may be wrong, but, I can't find a tabulated interaction for particle
wall in lammps. I am afraid some coding is needed.

correct. ​there isn't. but​ there *is* a 9-3 wall already available, but
the screened electrostatic (exponential) will need to be programmed. so
here is the chance for somebody's 15mins of fame.

axel.

Hi,

Thanks very much for your information.

Before I build my model, I need to understand the existing similar model.

In my case, the wall will be replaced by coarse grain carbon model and the lubricant (coarse grain) stay the same.
Under the carbon, there will be a substrate, thus that the carbon layer can move.

So, the actual question here is that, how can I distinguish the backbone and end beads?