[lammps-users] WCA potential

Dear Lammps Users,

I am have quick question regarding implementing WCA potential in lammps. For those who are not aware of it, It is essentially LJ 12-6 potential with additional epsilon added to it. It is generally used as a repulsive interactions with a cutoff at sigma*2^(1/6) (the distance at which interaction is negative epsilon). So basically the total interaction becomes zero at this distance.

I tried to look for various pair styles but I could find something like it. If I use lj/cut or lj/expand (for particles of different size) with the cutoff at sigma2^(1/6), I get the interaction energy as “-epsilon”. One option is to use pair_modify shift yes to move the interaction to zero. However, the problem with it is that it doesn’t modify the forces. So, for the distances between (sigma and sigma2^(1/6), the particles would feel attractive interaction).

This potential is often used to repulsive interactions between unlike atoms. It may be possible that I might be overlooking a particular pair_style.

I will highly appreciate if anyone could help me out here.

Vikas

Why do you think that particles will feel an attractive interaction? The
minimum of the Lennard-Jones potential is at 2^(1/6), so if you cut (and
shift) the potential you will keep only the repulsive part. Even though
the Lennard-Jones potential changes its sign at sigma, this is not the
case for the derived force.

Lutz

Hi Vikas,

I suppose that you just need to use pair_style lj/cut with the shift flag set to be yes. For the WCA potential between two types i and j, you then set the cutoff parameter to be 2^(1/6) = 1.122.

pair_style lj/cut 2.5
pair_modify shift yes

pair_coeff type_i type_j 1.0 1.0 1.122
# other pairs....

Hope this help,

-Trung

Quoting Vikas Varshney <[email protected]...>:

Dear Lutz, Joanne,
I think I was too quick to make conclusions from the equation. Thanks to both of you for clarification. You are both right. I should have thought a bit more before posting. :)…

Regards,
Vikas

LJ, cutoff at 2^(1/6), is exactly WCA. The shift by
epsilon affects bookkeeping for energy, but does not
affect forces or dynamics.

Steve