From: JC Gumbart (gumbart_at_ks.uiuc.edu)
Date: Fri May 25 2007 - 21:06:36 CDT
Without pressure control (langevinPiston off), the default behavior
is for the volume to remain constant.
Similarly, without temperature control, the default behavior is for
the energy to remain constant (within the limits of computational
algorithms, multiple time stepping for instance is known to introduce
an energy drift), provided there are no external forces applied.
This makes sense since Newton's equations provide no means for energy
to enter or exit the closed system, only to transfer between
potential and kinetic energy which is why the temperature can fluctuate.
Temperature control is implemented typically through Langevin
dynamics, which includes a damping force and a random force. These
two forces are external and thus cause energy to be put into or
removed from the system.
The moral of the story is that constant temperature and constant
energy are not compatible. The same can be said for volume and
pressure, among other pairs of variables.
On May 25, 2007, at 6:03 PM, Eric Perim wrote:
> Dear all,
>
> As far as I know, NAMD default is to carry NVE simulations, so
> if there is no Temperature or Pressure control on that is the
> ensemble which will be used, but I would like to know how I can,
> using only a temperature control, choose between NVT and NTE
> simulations, since I can't find any control to keep the volume or
> the energy constant. Up until now, every time I kept the
> temperature constant the volume didn't change, but last time I ran
> a simulation with the usual parameters the volume completely
> changed. Could anyone tell how to ensure that the volume will be
> kept constant?
>
> Thanks in advance,
> Eric Perim
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