[gmx-users] dg/dl unit

Berk Hess gmx3 at hotmail.com
Thu Nov 3 11:05:44 CET 2005

>Can I ask a naive question? Since when is lambda a unit? lambda is a 
>unitless quantity and quantities usually do not appear in a unit. If you 
>want to do it completely correct, I think that dG/dl has the unit kJmol-1.
>If you would do a slow growth simulation in which you modify lambda as a 
>function of time, then you probably write out dG/dl * dl/dtime and the unit 
>would be kJmol-1ps-1.

That is not a naive question.
I did not state my reply carefully enough.
The unit should be:
kJ mol-1 [lambda]-1
Where [lambda] means the unit of lambda.
Often lambda will be a dimensionless coupling parameter.

But lambda can have a unit.
I have done simulations where lambda=0 corresponds to a constraint distance
of 0 nm and lambda=1 to a constraint distance of 1 nm.
dG/dlambda is then the constraint force which has the unit:
kJ mol-1 [lambda]-1 = kJ mol-1 nm-1, which is indeed a force.


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