[gmx-users] System almost blowing up

Justin Lemkul jalemkul at vt.edu
Tue Apr 4 14:56:53 CEST 2017

On 4/3/17 2:33 PM, Dayhoff, Guy wrote:
>>> Hi,
>>>   I?m receiving a couple (~15) EM did not converge warnings as well as a few (~3) 1,4 interaction
>>> warnings during my run. It looks like it starts down the path to blowing up but recovers. Is this ?recovery?
>>> system dependent? Should I take these messages (even without the subsequent crash) to indicate an
>>> underlying issue with my system/topology or equilibration? I?m not ignoring or circumventing any warning
>>> during pdb2gmx, or any other commands.
>>> For more context: I?m running the Drude-2013 forcefield, with a dt of .5fs emtol of 1.0 and niter of 150
>>> using the V-rescale thermostat and Berendsen pressure coupling while my systems cell shape relaxes
>>> as a continuation from position restrained NVT then NPT ensembles. The starting structures were minimized
>>> in vacuum, then solvated and minimized once again prior to the posres equilibration.
>> Such a short dt and strict niter should not be necessary in practice.  The
>> failure of SCF to converge/LINCS warnings is what we typically refer to as
>> polarization catastrophe, so your system is on the brink of instability.  This
>> is one of the inherent problems of SCF in polarizable systems; it often fails to
>> converge and leaves the system potentially unstable.  The reflective hardwall is
>> much more reliable (and faster).  If the instability is happening in your
>> equilibration, that may be OK but it is something you should try to troubleshoot
>> to make sure there's nothing else going wrong.
>> -Justin
>> --
> Got it. I have been spending some time investigating the issue i’m having and
> after reading shellfc.cpp I was pretty confident the issue was with SCF not converging,
> and so my niter and dt were an attempt at looking into that.
> The workflow i’ve been trying to use is…
> EM -> Solvate -> EM -> posres NVT V-rescale -> posres berendsen NPT isotropic 1bar w/ V-rescale tcoupl ->
> berendsen NPT semi-isotropic various pressures w/ V-rescale tcoupl
> then once my cell shape has reached an equilibrium I finally switch to NH thermostat and PR barostat for
> the production run.
> If I switch to employing the hardwall (and the lagrangian dynamics for drudes) then I understand it is not
> compatible yet with NPT ensembles. So, if I change my work flow as follows would it be appropriate still?
> EM->Solvate->EM->posre NPT isotropic 1 bar w/ V-rescale tcoupl->NPT semi-isotropic varying pressures
> w V-rescale tcoupl until cell shape is equilibrated using PR barostat, then switch to NVT without pcoupl
> to use the hardwall during my production run?
> For some reason, i’ve got this idea that I need to be tcoupling and pcoupling during my production runs
> so i’ve been apprehensive to do this.

If you want an NPT ensemble during dynamics you're going to have to use NAMD, 
CHARMM, or OpenMM.  I haven't coded the barostat yet (fixing the DD bugs has 
been more important).  You can equilibrate with SCF under NPT and get a suitable 
density, then switch to NVT, but your use of semi-isotropic coupling suggests a 
membrane system?  In that case, I would not use NVT during production as you 
will be simulating a constant area, which may not be appropriate.



Justin A. Lemkul, Ph.D.
Ruth L. Kirschstein NRSA Postdoctoral Fellow

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 629
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441


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