# About Modify LJ 126 Potential In LAMMPS

Dear All,

I want to modify the LJ 126 potential to 4epsilon(2*(sigma/r)^12-2^(1/2)*(sigma/r)^6).

Can I do this way:

pair_style lj/cut 2.5
pair_coeff * * 1.0 2^(1/12)

Best Wishes!

You can validate it yourself by expanding the terms.

Francis

The other popular choice is
U® = epsilon * ((sigma/r)^12 - 2*(sigma/r)^6)

If this is what you want, then you are correct that all you have to do is decrease your “sigma” by 2^(-1/6). This is because “sigma” in the formula above is proportional to the sigma (used in 4eps((sig/r)^12-(sig/r)^6 ).

sigma_in_12formula = sigma_used_by_lammps * 2^(1/6)

You can also check out:

http://www.moltemplate.org/lammps_code/pair_lj_charmm_coul_charmm_inter.html

(however there is currently a bug when the inner and outer cutoffs are different for coulombic and lennard jones forces)

But to solve the problem you asked about, I think it’s much simpler just to multiply sigma by 2^(-1/6).

Cheers

Andrew

Thanks very much!

This is exactly what I want.

I didn’t check 2^-(1/6) to get just the attractive part of LJ.

Best Wishes!