Hi all,

I am just working on a system consisting of two types of particles with different shapes. The first kind is a simple sphere with radius one and the other one is an ellipsoid with radii 3 1 1. The potential I’m going to use is gayberne. I am confused setting the potential parameters. I have read the manual but I think it contradicts with lammps example “ellipse”. I don’t know what is the relationship between potential parameters and desired ellipsoid radii.

Let an ellipsoid have radii a b and c. Consider the following lines:

shape a b c

…

pair_coeff 1 * epsilon sigma x1 x2 x3 y1 y2 y3

How should I set epsilon, sigma, x1, x2 and x3 in order to apply the different radii of ellipsoid in potential?

Thank you in advance

M. Baghery

Hi all,

mehrdad,

I am just working on a system consisting of two types of particles

with different shapes. The first kind is a simple sphere with radius

one and the other one is an ellipsoid with radii 3 1 1. The potential

I'm going to use is gayberne. I am confused setting the potential

parameters. I have read the manual but I think it contradicts with

lammps example "ellipse". I don't know what is the relationship

between potential parameters and desired ellipsoid radii.

Let an ellipsoid have radii a b and c. Consider the following lines:

shape a b c

...

pair_coeff 1 * epsilon sigma x1 x2 x3 y1 y2 y3

How should I set epsilon, sigma, x1, x2 and x3 in order to apply the

different radii of ellipsoid in potential?

i would rather say that _your_ example contradicts bother

the ellipse example and what is in the LAMMPS manual.

the syntax for shape is:

shape <type> <a> <b> <c>

i.e. it takes four arguments and similarly there are

no x1 x2 x3 y1 y2 y3 parameters for the pair_coeff

command. in my (up-to-date) version of LAMMPS the ellipsoid

example matches the documentation well.

what version of LAMMPS are you using?

cheers,

axel.

I talked to Mike Brown about this. The GB standard practice

is to define sigma as the min of the 3 shape diams, though

you don't have to. This is different than the RE-squared

potential where sigma is the size of the small sphere you

are integrating over, typically 1 regardless of the size

of the much bigger ellipsoid.

Note that for I,J interactions sigma can be mixed or

specified directly with a pair_coeff command.

I've amended the doc pages to reflect this guidance.

Steve