[CP2K:64] practical MD-barostat question

Nichols A. Romero naro... at gmail.com
Thu May 10 19:15:51 CEST 2007


Tell us a little bit more about the method for the FORCE_EVAL. I work mostly
with QS (DFT) and one has to work hard to get accurate pressures in general.
An accuracy below 0.1 GPa, 1000 bar is non-trivial (probably not even
realistic given other sources of systematic error).

1 bar is a tiny pressure. I would say that with the thermostats the most
important thing to check are the averages of course. 5000 bar is 0.5 GPa. I
think that's reasonable. If you have plenty of computer resources one thing
you could check is how the fluctuations change with system size. In NVT for
example, I believe it can be shown that the fluctuations in T should scale
as 1/sqrt(N). Don't know if this is true for other ensembles.

On 5/10/07, Toon <Toon.Ver... at gmail.com> wrote:
> Hi,
> When performing an MD simulation, using the NPT_I ensemble with the
> default settings and a target pressure of 1 bar, what is the order of
> magnitude of the oscillations in the instantaneous pressure that one
> should expect? (I have set PV_AVA to true in the input file to get the
> instantaneous pressure in the output file.) Since I'm new on this
> subject, I was performing a simple test run on a box of solid methane
> with 64 molecules. (low temperature, below 90K, zero diffusion) I see
> oscillations in the instantaneous pressure with an amplitude of 5000
> bar. Is this normal? Maybe the answer is very system dependent?
> best regards,
> Toon
> >

Nichols A. Romero, Ph.D.
1613 Denise Dr. Apt. D
Forest Hill, MD 21050
443-567-8328 (C)
410-306-0709 (O)
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