[gmx-users] Water molecules cannot be settled: V-rescale is the cause

[Ladasky]

After weeks of trying various conditions, I found my problem.  Here, from my
position-restrained MDP file, are the two relevant lines:

; Temperature coupling is on 
tcoupl            = V-rescale ; Weak coupling for initial equilibration 
[snip]
; Pressure coupling is on 
pcoupl        = Berendsen     ; Pressure coupling on in NPT, also weak
coupling 

This is a change that I forgot to mention in my list of changes, but in fact
I did post the full MDP file.  I switched to using two DIFFERENT algorithms
for temperature and pressure coupling when I performed the initial
position-restrained equilibration.  This frequently sets up an INSTABILITY
which manifests itself anywhere from a few picoseconds to a full nanosecond
into the standard equilibrium MD run.  

I am giving this number in real time, rather than in compute cycles, because
I tried cutting my step time to 1.0 fs from my usual 2.0 fs -- and I still
had a water molecule dislocate abruptly, at nearly the same physical
location and simulated time point.

I will share my full debugging process if anyone is interested.  It took me
several runs, all of them over 24 hours long, to figure this out.

Now, WHY would I have switched from a Berendsen temperature-coupling
algorithm to the V-rescale algorithm?  Because of this cautionary note that
I started receiving in GROMACS 4.5 when I started the position-restrained
mdrun:

NOTE 1 [file pr.mdp]:
  The Berendsen thermostat does not generate the correct kinetic energy
  distribution. You might want to consider using the V-rescale thermostat.

I took this as a warning that a deeper study of the algorithms had revealed
a flaw in Berendsen, and so I sought to use what GROMACS was advising me
would be a better choice.  This is apparently MISLEADING advice when doing
the initial equilibration.  If you are using the Berendsen pressure
algorithm, you must also use the Berendsen temperature algorithm.

I know that mdrun cannot be sophisticated enough to know which phase of my
run I am performing, but I hope that the GROMACS documentation will be
revised to discuss the appropriate use of the V-rescale thermostat.  I just
misused it,and wasted a lot of time as a consequence.  I hope that some
discussion of its proper use would be written and summarized on a page like,
for example, this one:

http://www.gromacs.org/Documentation/Terminology/Equilibration

It already states: "Using, i.e., the Nosé-Hoover thermostat for initial NVT
equilibration can lead to wild oscillations of the temperature, with the
system ultimately blowing up."  A similar warning should be added about the
V-rescale thermostat in NPT equilibration, ESPECIALLY if GROMACS is going to
issue messages that recommend that you use it!

There is a page for the Berendsen barostat/thermostat:

http://www.gromacs.org/Documentation/Terminology/Berendsen

There is apparently no corresponding page for the V-rescale thermostat. 
Perhaps one should be written.




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