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[ESPResSo-users] Dipoles -- Was: Bug in python interface?
From: |
Rudolf Weeber |
Subject: |
[ESPResSo-users] Dipoles -- Was: Bug in python interface? |
Date: |
Tue, 23 Jan 2018 14:08:53 +0100 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
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