[gmx-users] REMD problem in concept

Mark Abraham Mark.Abraham at anu.edu.au
Tue Dec 2 14:51:44 CET 2008

sarbani chattopadhyay wrote:
>   Hi everybody,
>                       I am having a doubt whether I have understood the 
> concepts of Replica
> exchange molecular dynamics correctly or not.
>                       When a pair of replicas are exchanged, the one at 
> higher temperature thet has
> higher velocities, (which are rescaled after the exchange) comes with a 
> higher energy. It's
> kinetic energy gets reduced as velocities are rescaled. Thus its 
> potential energy increases.

Kinetic energy is a function of the velocities, potential energy is a 
function of the positions. So velocity rescaling will not have any 
instantaneous effect on the PE.

> What happens to the replica that was at lower temperature? It's 
> velocities need to be increased
> to maintain the temperature. What happens to it's potential energy then?

As before, nothing.

> I may be missing a very basic point and any suggestion regarding this 
> will be highly helpful.

Once the simulation continues, things are still normal. The reason the 
exchange occurred was that the positions were such that the PE was close 
enough that detailed balance for the generalized ensemble was preserved 
  during the swap. Thus the ensembles before and after the exchange are 
also still well-formed.


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