[gmx-users] friction constant for free energy calculation

Justin Lemkul jalemkul at vt.edu
Thu Feb 7 19:49:18 CET 2019

On 2/6/19 7:28 AM, minky son wrote:
> Dear gmx users,
> I was trying to calculate free energy.
> I’m confused about setting the appropriate value for friction constant (=
> 1/tau-t).
> In gmx manual, a value of 0.5 ps-1 (tau-t = 2.0 ps) was suggested
> but 1.0 ps-1 (tau-t = 1.0 ps) was used in Justin’s tutorial.

I was reproducing a published paper, so I used the same value. The 
manual is not an authoritative source of "do this all the time," though 
there are default or example values that are usually functional.

IIRC, when using sd, tau-t is read directly as ps^-1 and not inverted. 
You'd have to check the code to be sure.

> How much the results are different by the use of tau-t value 1.0 or 2.0

Likely very little. The friction coefficient affects dynamics, but not 



Justin A. Lemkul, Ph.D.
Assistant Professor
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