[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
## Re: Gamma in Langevin Thermostat

**From**: |
Rudolf Weeber |

**Subject**: |
Re: Gamma in Langevin Thermostat |

**Date**: |
Fri, 24 Apr 2020 19:39:25 +0200 |

**User-agent**: |
Mutt/1.9.4 (2018-02-28) |

Hi,
On Fri, Apr 24, 2020 at 05:08:53PM +0000, Ahmad Reza Motezakker wrote:
>* I hope you all are fine and healthy.*
>* *
>* I have been trying to understand how I can set gamma parameter in langevin *
>* thermostat. I have gone through the online documentation, github, and *
>* previously asked questions in espressomd archive with no clear answer. I will *
>* be really thankful if you open it up for me. I have a single particle with *
>* mass of 8*10^(-18) grams (which is not considered in the simulation but of *
>* course for doing unit conversions and calculating other parameters such as *
>* time unit) at room temperature.*
>* *
>* Thank you again.*
The equation of motion being solved is
mx.. = -gamma x. +F(x,x.) +F_random
Here, x = position, x.=vleocity, x..=acceleration.
F(x,x.)=forces from your potentials
F_random = Random kicks applied by the thermostat.
All terms in the equation are of unit force.
So, for the term including gamma we have [gamma_unit] * [velocity] = [force]
So, gamma would be in Netwon second / meter, if I didn't miscalculate.
All of this is only relevant, if you actually want to look at time-dependet
behavior. Otherwise, gamma can be chosen for stability of the integration.
For a Lennard-Jones system with LJ epsilon=1 and sigma=1 at a thermal energy
kT=1, values between 1 and 10 will do for gamma. Use 1 for the particle mass,
in that case.
Hope that helps.
Regards, Rudolf