[lammps-users] Ellipsoidal shaped particles in dusty plasma

Hello!
I want to study the behavior of ellipsoidal shaped particles in dusty
plasma using lammps. I want to make a hard ellipsoid so that they come closer upto a certain distance as i am using coul/debye potential. Is it possible using this potential ?. But when I am using script (as follows) and I visualized these particles using OVITO software I got these ellipsoidal particles are crossing each others and the orientation of ellipsoidal particles is also not changed.
I also attached the lammps script which I used

units si
atom_style hybrid charge ellipsoid
dimension 2
boundary p p p

#Atom definition

region simbox block 0 {lxx} 0 {lyy} -1e-4 1e-4
create_box 1 simbox
create_atoms 1 random ${Np} 3432500 NULL

set group all shape 3 2 1
set group all quat/random 253040
set group all charge {Qd} set group all mass {md}

you need to have something that imposes a torque on your particles, but your only interactions are due to the could/debye pair style, which is a spherical potential and thus only creates forces on the center, but no torque. hence your ellipsoids will not change their orientation.’

axel.

Thank you

Axel Kohlmeyer It really helpful for me. I have one more question I also make spherical particles using this coul/debye potential in lammps

I attached the script and video here. I have two spheres of the same mass and opposite charge (charges same in magnitude but opposite in sign) with no external force. so instead of repealing each other, they cross each other and after that, they get separated. my guess was they get repelled by each others when they come closer to each other. I would be very grateful if you can help.
PFA
script as follows

units si
atom_style hybrid charge sphere
dimension 2
boundary p p p

query.mp4 (107 KB)

Please keep in mind that this is a new question and thus mailing list etiquette stipulates that you should write a new email with a new subject instead of replying to a previous email.

Please also do not attach large binary blobs to mailing list emails unless asked to. Your mail is copied to over 1300 people and you are just wasting their storage and network bandwidth. This is particularly true for videos. It is much more effective to describe the situation and provide the (complex) input deck and command line so that people can easily reproduce the trajectory if necessary.

To answer your question, why should those atoms repel each other. Last time I checked the Coulomb potential it would cause that opposite charges are attracted to each other. And that is the only interaction you have.

It looks to me you need to work some more on basic theory of MD simulations before running actual simulations.

Axel

Please keep in mind that this is a new question and thus mailing list etiquette stipulates that you should write a new email with a new subject instead of replying to a previous email.

Please also do not attach large binary blobs to mailing list emails unless asked to. Your mail is copied to over 1300 people and you are just wasting their storage and network bandwidth. This is particularly true for videos. It is much more effective to describe the situation and provide the (complex) input deck and command line so that people can easily reproduce the trajectory if necessary.

To answer your question, why should those atoms repel each other. Last time I checked the Coulomb potential it would cause that opposite charges are attracted to each other. And that is the only interaction you have.

It looks to me you need to work some more on basic theory of MD simulations before running actual simulations.

Axel is just pointing out that you need (forgot?) to add short-range repulsive interactions between your particles. For ellipsoidal particles, you would typically use “pair_style gayberne”. However if you want to add charge, then you need to use a “hybrid/overlay” pair style to combine the electrostatic and short-range repulsive forces:

pair_style hybrid/overlay gayberne coul/debye 10.0

or
pair_style hybrid/overlay gayberne coul/long 10.0

For details, see these links:
https://lammps.sandia.gov/doc/pair_coul.html

https://lammps.sandia.gov/doc/pair_gayberne.html

https://lammps.sandia.gov/doc/pair_style.html

(Note: There is a way to shift the charge away from the center of the ellipsoid using “pair_style coul/long/offcentre”, which is included with the MOLC source code. That code is included in the supplemental materials of this paper. Hopefully they will submit their code so that it can be included with LAMMPS.)

In ordinary simulations that do not use ellipsoidal particles, the short-range (Lennard-Jones) repulsive forces and electrostatic forces are typically combined into the same pair style, for example:

https://lammps.sandia.gov/doc/pair_lj_cut_coul.html

I hope this helps.

Andrew

I forgot to mention that for ellipsoidal particle simulations, I recommend using OVITO instead of VMD to look at your simulations. (And maybe for other simulations too.)

If it helps, Otello Roscioni and Matteo Ricci (MaterialsX LTD) created a (very short) README file and a screenshot image to get you started using OVITO with ellipsoidal particles.

https://github.com/jewettaij/moltemplate/blob/8ee4237debf8b7f889536c9bc9fee1de87235e92/examples/coarse_grained/MOLC/README_visualization_OVITO_settings.png

https://github.com/jewettaij/moltemplate/blob/8ee4237debf8b7f889536c9bc9fee1de87235e92/examples/coarse_grained/MOLC/ellipsoids/README_visualization_OVITO.txt

There are probably better introductions to OVITO available. But hopefully this gets you started.

Cheers

Andrew