From: Richard Law (rlaw_at_llnl.gov)
Date: Fri May 25 2007 - 22:27:39 CDT
There was a discussion of this before:
On 5/25/07, JC Gumbart <gumbart_at_ks.uiuc.edu> wrote:
> 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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