[Patch Particle Simulation] How can I get the orientation of a particle

[유지웅]

Dear HOOMD users,

I want to do a patch particle simulation.
The system I'm trying to simulate consists of isotropic spheres with anisotropic force field.
To be more specific, I want to use Kern-Frenkel model(Link: http://aip.scitation.org/doi/10.1063/1.1569473 ).
I think I need orientation of each particle which is included in HOOMD in the form of quaternion to use this potential.
However, I cannot find how to get this quaternion to make angle dependent potential in Kern-Frenkel model.
Is there any feature or manual to follow to make this angle dependent potential?
Or can I get an access to quaternion which is updated every single run for the pair potential to use? 

Thanks in advance!

Jiwoong, Yu

[Nathan Melton]

I'm interested to know if this can be done as well.

[Michael Howard]

What you are describing is an anisotropic pair potential. There are some of these already implemented in HOOMD, see for example md.pair.dipole (http://hoomd-blue.readthedocs.io/en/stable/module-md-pair.html#hoomd.md.pair.dipole). You would need to write a plugin implementing the specific functional form for the potential you have in mind, which computes the necessary forces and torques on the particle. You can again use the dipole as an example, looking at hoomd/md/EvaluatorPairDipole.h and its associated files as an example.

--Mike

P.S. As I'm sure you know, the Kern-Frenkel model is a discontinuous potential, and so it will need to be modified for use in MD.

[Joshua Anderson]

Kern-Frenkel is discontinuous.  We have implemented a smoothed out version of it in an old hoomd plugin to model Janus particles: http://dx.doi.org/10.1039/c3sm53136h

I will contact the developers of the code and see if we can get it updated for HOOMD 2.x or at least make it available so that an interested community member could update it for 2.x and contribute it to the project.

------
Joshua A. Anderson, Ph.D.
Research Area Specialist, Chemical Engineering, University of Michigan
http://www-personal.umich.edu/~joaander/

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