[gmx-users] Water molecules cannot be settled: V-rescale is the cause
blind.watchmaker at yahoo.com
Wed Sep 19 23:10:08 CEST 2012
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
; Pressure coupling is on
pcoupl = Berendsen ; Pressure coupling on in NPT, also weak
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
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:
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:
There is apparently no corresponding page for the V-rescale thermostat.
Perhaps one should be written.
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