# Repulsive-only Gay-Berne Potential

Hello,

I was trying to setup a MD simulation for aspherical particles using Gay-Berne potential (U_GB).

My interest is in soft-repulsive aspherical particles, which will have interaction of the form

U_rep_GB = U_GB - U_GB^min for r < rc(u1,u2)
else zero

For different configuration/alignment of the particles (side-to-side, end-to-end etc), the cutoff of
the potential varies. As of now in LAMMPS, I can give either a global-cut off or a cut-off in
pair_coeff. I have tried reading manual page for the pair_modify and found this works only for
sphere-sphere interaction. Is there any other way to do this in LAMMPS.

Regards
Vishwas

The pair gayberne doc page tells you what all its parameters
are, including the cutoff. Whether you can do what you are asking
depends on those formulas. There is no other magic in LAMMPS
that will figure out the relative orientiation between two ellipsoids,
other than in that pair style at the moment those eqs are computed.

Steve

Hello Steve,

In case of simple LJ if I choose the cut of to be 2^(1/6), I would have only repulsive part. For aspherical particle using GB potential, (say for mu=2, nu=1, k=3, k’=5), the side-side configuration profile would have minimum of potential around rij=1.2, the end-to-end configuration would have minimum of the potential at around rij=3. If I were to make repulsive-only GB interaction, I can not simply choose a single cut off for the potential. This is what I was mentioning in my previous mail.

Regards
Vishwas

Hello Steve,

Just to be more clear, I am attaching the GB potential profile. If you see the potential profile, you will find that the minima of the potential varies with alignment of GB particles (or rod like particles). If I were to make repulsive-only GB potential what should I be doing was my question.

Regards

Vishwas

GB_Profile.eps (30.3 KB)

Hello Steve,

Just to be more clear, I am attaching the GB potential profile. If you
see the potential profile, you will find that the minima of the potential
varies with alignment of GB particles (or rod like particles). If I were to
make repulsive-only GB potential what should I be doing was my question.

this is a superfluous question. if you would spend just a little bit
of time considering your options, there is only one option: you have
to program a customized gay-berne pair style. the existing parameters
don't offer the facility that you are looking for, so you will have to
add it. plain and simple.

axel.

Said another way, the GB potential apparently
does not do what you want (repulsive only for
all orientiations). So you need to create your
own new potential. That is typically a bad idea
since parameterizing it to match desired material
properties is a lot of work, and requires some

expertise.

Steve