|Subject:||[ESPResSo-users] langevin gamma not friction coefficient in v3.3.1?|
|Date:||Sun, 15 Jan 2017 11:59:05 -0500|
|User-agent:||Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1|
According to another thread in this archive - http://lists.nongnu.org/archive/html/espressomd-users/2015-11/msg00019.html - the parameter gamma in the Langevin thermostat represents a friction coefficient, such that F_friction = gamma*v (whereas in earlier versions, it used to be a relaxation time: F_friction = gamma_old *m * v). The thread was posted a few months after the release of the latest Espresso version (3.3.1), so I assumed it refers to that version.
However, I'm using v3.3.1 and from what I can tell, it seems that
gamma is not a friction coefficient here, because the friction
force still has a mass term in it. For example, see the attached
sample script, where I compare the velocity decay of two particles
of different masses. The profiles are identical, which would only
happen if the mass term in friction cancels out the mass term in
Moreover, looking at the source code, I think this is the relevant bit for the friction term (in the file thermostat.hpp): p->f.f[j] = langevin_pref1_temp*p->m.v[j]*PMASS(*p)+ ... It seems like there is a mass term in there. This is in contrast to the development code on github, where the equivalent line reads p->f.[j] = langevin_pref1_temp[j]*velocity[j] + ...
Is this assessment correct? If so, and if I want gamma to be a friction coefficient, is the solution as simple as removing the mass term in thermostat.hpp, or should I just use the development version?
Description: Tcl script
|[Prev in Thread]||Current Thread||[Next in Thread]|