[ESPResSo-users] Dipoles -- Was: Bug in python interface?

[Rudolf Weeber]

Hi Andreas, 
> thanks a lot - this helped to get a running version.
> However, I still have trouble with setting up dipolar particles using
> virtual sites: a hard sphere and two virtual particles carrying charges.
> Do you know about any examples? The particle set up time always explodes
> (not finishing any more) after I set one of the charged particles virtual.
A potential issue is charges getting too close to each other. Also, 
system.analysis.min_dist() includes virtual sites, so It might never get 
comparable to sigma, unlkess you restrict the particle types it acts on or the 
like.

I'd suggest the following order
1. Setup the central (non-virtual) particles with its Lennard-Jones potential 
and warmup via the steepest descent integrator (or force capping) to remove any 
overlap.
See 
espressomd.org/html/doc/espressomd.html#espressomd.integrate.Integrator.set_steepest_descent
2. Integrate some more with the Langevin thermostat (without force capping) to 
randomize positions and orientations.
3. Place the two virtual sites at
vec(r_vs) =vec(r_central) +,- l/2 *vec(director)
where director is the director property of the central particle (z-direction in 
its body frame expresed in space frame). 
note that the length of your dipole (l) needs to be smaller than sigma or two 
charges will go on top of each other.
(testsuite/virtual_sites_relative.py contains a slightly related setup for 
dumbbels, i.e., two overlapping Lennard-Jones spheres in the function 
run_test_lj())
4. Assign the charges to the virtual sites and activate the electrostatics 
method.
5. Integrate some more with the Langevin thermostat to equilibrate with 
electrostatics.

BTW: If you need only dipole-dipole (as opposed to dipole-charge) interactions 
and point-dipoles, you can use Espresso's magnetostatics features. That's what 
we are using for ferrofluids and the like.

Regards, Rudolf

--

Dr. Rudolf Weeber
Institute for Computational Physics
Universität Stuttgart
Allmandring 3
70569 Stuttgart
Germany
Phone: +49(0)711/685-67717
Email: address@hidden
http://www.icp.uni-stuttgart.de/~icp/Rudolf_Weeber