[gmx-users] System blowing up - equilibration step - CG simulation

[Justin Lemkul]

On 3/6/15 8:15 AM, Carlos Navarro Retamal wrote:
> Dear Gromacs users,
> I'm trying to perform a CG simulations of a system consisting in a protein locate at 5 nm with respect to the charged groups of a MGDG membrane.
> I first performed an EM in vaccum.
> After that, to constructed the system i use insane.py as following:
> ./insane.py -f minimization-vaccum.gro -o system.gro -pbc square -dm 5 -box 10,10,10 -l MGDG -sol W -orient
>
> Later, i ran a EM with the whole system, with the respective output:
> Steepest Descents converged to machine precision in 2081 steps,
> but did not reach the requested Fmax < 10.
> Potential Energy = -2.6833109e+05
> Maximum force = 3.5193942e+02 on atom 3598
> Norm of force = 6.2432761e+00,
>
> Sadly, when i'm trying to perform an equilibration i got the following error message:
> Step 10:
> Atom 8449 moved more than the distance allowed by the domain decomposition (1.590000) in direction Z
> distance out of cell 1330.886108
> Old coordinates: 1.667 4.809 0.710
> New coordinates: -1840.027 -454.814 1335.667
> Old cell boundaries in direction Z: 0.000 5.000
> New cell boundaries in direction Z: 0.000 4.781
> ________________________________
> Program mdrun, VERSION 5.0.4
> Source code file: /home/krlitros87/Downloads/gromacs-5.0.4/src/gromacs/mdlib/domdec.c, line: 4390
>
> Fatal error:
> An atom moved too far between two domain decomposition steps
> This usually means that your system is not well equilibrated
> For more information and tips for troubleshooting, please check the GROMACS
> website at www.gromacs.org/Documentation/Errors<http://www.gromacs.org/Documentation/Errors>
>
>
> This is the .mdp file:
> define = -DPOSRES
> dt = 0.02
> cutoff-scheme = group
> nsteps = 500000
> nstxout = 0
> nstvout = 0
> nstlog = 100
> nstxtcout = 100
> xtc-precision = 10

Any reason to sacrifice so much precision?  Maybe for a CG system it doesn't 
matter so much.

> rlist = 1.4
> coulombtype = shift
> rcoulomb = 1.2
> epsilon_r = 15
> vdw-type = shift
> rvdw-switch = 0.9
> rvdw = 1.2
> tcoupl = Berendsen
> tc-grps = Protein MGDG W ION

Coupling water and ions separately is generally unstable.

> tau-t = 1.0 1.0 1.0 1.0
> ref-t = 323 323 323 323
> Pcoupl = Berendsen

If NPT is failing, reduce complexity and start by equilibrating with NVT.

> Pcoupltype = isotropic
> tau-p = 5.0
> compressibility = 3e-4
> ref-p = 1.0
> refcoord_scaling = all

Try refcoord-scaling = com; the all option can be unstable.

>
> I know that my system is blowing up, but what can i do to avoid this issue? I tried increasing the EM step without luck.

What about all the rest of the advice at 
http://www.gromacs.org/Documentation/Terminology/Blowing_Up?  Your EM seems 
fine, so I doubt that's it.  Something is either unstable in the topology or the 
dynamics.  Split the system into parts to make sure you can stably simulate 
everything - protein in water, membrane alone, etc.

-Justin

-- 
==================================================

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
http://mackerell.umaryland.edu/~jalemkul

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