[gmx-users] Re: energy terms vs. temperature

Justin A. Lemkul jalemkul at vt.edu
Wed Sep 7 01:59:03 CEST 2011

Dr. Vitaly V. Chaban wrote:
> Hello Juliette:
> If you observe such behavior, I would suggest just to decrease a
> time-step. You should have the same energies at all temperatures.
> The larger is a time-step, the larger is a deviation from the energy
> minimum (bonded potentials). The larger is the deviation, the higher
> energy/force arises in order to get back to the equilibrium.

Shorter time steps may be necessary to maintain stability of the system and to 
conserve energy, certainly, but I don't understand your point about the energy 
being independent of temperature.  Increasing temperature should, in theory, 
lead to excited states for bonds so they should be of higher energy.  For NVE, 
temperature fluctuations and compensatory potential changes keep the energy 
constant, but at different temperatures, NVT and NPT ensembles will inherently 
have different energies (and techniques like REMD derive their usefulness in 
part from this fact).

A given system should have a constant total energy, but I would certainly expect 
a system run at 300 K to have a different total energy from one at 400 K.



Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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