[lammps-users] Is GB potential so soft?

Dear all,

I run a simple bulk with gay-Berne ellipse.

After coarse minimize, npt and nve computation, the trajectory show that these ellipse can interpenetrate with each other. It is very strange because the GB potential graph of some articles showed the change tendency similar to VDW potentials.

My parameters setting for GB( see/sss, eee/ess, n,m) GB (4.4,20,1,1) ellipse is :

shape 1 1 1 4.4 # name a b c

pair_style gayberne 1.0 1.0 1.0 5.5 # type g n m cutoff

pair_coeff 1 1 1.0 1.0 1 1 0.05 1 1 0.05 # name_i name_j e s sia sib sic sja sjb sjc

And I also found that there is no energy decrease in minimization.

I am eager for your help.

Here was Mike Brown's answer to these issues:

I took a look at your log file. There are 2 issues:

1. Although the periodic lattice you have created is a high energy
configuration due to the overlapping ellipsoids, the configuration is at
a maximum. This is because each ellipsoid can spin either direction
around an axis to alleviate the overlap (or translate). Therefore the
torques (and forces) are 0.

2. I don't think that rotational degrees of freedom are incorporated
into the minimization routine yet (Steve can correct me if I'm wrong
here). If this is the case, it will only minimize position and not

There are several options that might help you out:

- Run MD with random velocities
- Start with a more dilute lattice and use either NPT or fix deform
along desired axes to shrink the simulation box and pack the ellipsoids.

Hope this helps.

- Mike