[gmx-users] about periodic image violations

Tue Jun 28 12:10:44 CEST 2011

Sorry, I forgot to change the subject of my previous mail.
Anna 

-----Messaggio originale-----
Da: Anna Marabotti [mailto:anna.marabotti at isa.cnr.it] 
Inviato: martedì 28 giugno 2011 12.09
A: 'gmx-users at gromacs.org'
Oggetto: R: gmx-users Digest, Vol 86, Issue 183

Dear Tsjerk, dear all,
thank you very much for suggestion. One of the main reasons why I never used
the rhombic dodecahedron box for my simulations is the fact that it is
really hard to visualize the results with programs such as VMD or Pymol, and
therefore I cannot check easily during the setting of my system if the work
proceeds properly. Since I would like to redo my simulations following your
suggestion, could you please give me some hints on how to proceed in order
to visualize easily the system? For example, now I created a dodecahedric
box filled with water and ions. I tried to visualize it with VMD and I saw
it as a rectangular box with the protein at one edge. I tried to use the
command trjconv:

trjconv -f prot_dod_box_neu.gro -s prot_dod_box_neu.tpr -o
prot_dod_box_neu2.gro -pbc mol -ur compact

and I obtained an error message stating:
Fatal error:
Index[84328] 84329 is larger than the number of atoms in the trajectory file
(84328)

I don't find this error neither in the gmx-users list archive (indeed it's
becoming harder and harder to search into this archive, given that each
message is visualized 10 times, as I already noticed sometimes ago), nor in
the Error section. The first .gro file is the one coming from genion, and
the .tpr file is the one I used to neutralize the system, which is the .tpr
file I should give to trjconv? You see, if every time I try to use a box
different from the standard cubic one I has to deal with such maybe trivial
(for you) problems, but difficult and long to solve, I'm quite discouraged
to use it! This is also a kind request for Gromacs Web site managers: could
you please add a section for this (recurrent, I suspect) problem of box
visualization in the "How-to" section? it would be very useful for users, I
think, and probably also for you (less trivial questions posted to the
gmx-users list...)

Many thanks in advance for the help you could provide me.
Anna

Message: 6
Date: Mon, 27 Jun 2011 17:05:17 +0200
From: Tsjerk Wassenaar <tsjerkw at gmail.com>
Subject: Re: [gmx-users] about periodic images violation
To: Discussion list for GROMACS users <gmx-users at gromacs.org>
Message-ID: <BANLkTim3r1339YB6ECKEDF9eTYRg+q+MqA at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

Hi Anna,

The spikes you see occur because the protein is broken over the
periodic boundaries. Not hard to see that a broken molecule will have
a minimal minimal distance.

The other problem may well occur due to rotation of your molecule.
Since you set -bt tric, you just get a rectangular unit cell, which
has the drawback that the shortest distance may not be long enough to
keep the protein from self-interactions in certain orientations. I
know it's considered a pretty standard setup, which is why I mentioned
it before. You should use a rhombic dodecahedron.

With a rectangular cell, you could even have self-interaction in a
nastier way: the ends of the protein can try to avoid each other,
rather than align with each other. That would result in an altered
ensemble, yet one in which you wouldn't see violations of the minimal
distance. In a rhombic dodecahedron the spatial distribution is much
more uniform...

Cheers,

Tsjerk

On Mon, Jun 27, 2011 at 12:48 PM, Anna Marabotti
<anna.marabotti at isa.cnr.it> wrote:
> Dear gmx-users,
> I'm inserting into the discussion about periodic images since I'm
> experimenting a problem of minimum distance violation too. I'm doing
> simulations on a dimeric protein (with no covalent bonds between the two
> subunits) which derives not from a crystallographic structure but from a
> model. I made several simulations changing the gen_seed in order to
explore
> deeply the conformational space of the protein. I used a triclinic box
> (option editconf -bt triclinic -d 1 -c) filled with spc water and
> neutralized with ions; in my opinion, it's quite a standard system. I
> equilibrated the system with a minimization (emtol = 500 reached) and with
> NVT+NPT in position-restrained mode (20+100 ps) at 310 K, then launched
the
> full MD for 30 ns, always at 310 K. I repeated this procedure
> (NVT+NPT+FullMD) for each of the gen_seed assigned (random numbers),
> starting from the same minimized structure. Obviously, in PR-NPT and full
Md
> simulations, I did not recalculated the initial velocity (it's a
> continuation). Before starting with full MD I checked for energetic
> parameters in .edr file (Pot, Kin, Tot, T, P) and all seem stable with no
> apparent problems. I run the 30 ns simulations and now I'm checking for
the
> results.
> Looking at the trajectories I see that in some (but not all) cases, the
> protein moves from the center of the box to one of the edges, starting
from
> a time that is different in each simulation, when it happens. I run
> g_mindist, and in some cases (generally when the protein doesn't move so
> much) I see some "spikes" in the plot (that disappear if I apply trjconv
> -pbc nojump to the trajectory), but apart from these "spikes" the minimum
> periodic distance in the trajectory is at least 1.5 nm (I set van der
Waals
> cut-off at 1.4 nm). In other cases, however (essentially when the protein
> starts moving towards the edges of the box), the minimum periodic distance
> starts decreasing (in some simulations after 10 ns, in some simulations
> after 20: there is not a common point after which you can see a sudden
> decrease of minimum periodic distance of the system) and reaches a value
> below 1.4 nm or even below 1 nm. Considering that the starting system is
> more or less the same in all cases, I don't identify the reason why the
> system "behaves" like this, and moreover what can I do to avoid this. My
> questions are:
> - do I have to enlarge the box? but I don't think that this would solve
the
> "motion" of the protein towards one edge of the box
> - do I have to change the box? In his last message Tsjerk suggests to use
a
> rhombic dodecahedron box, could it be useful in my case?
> - do you think it's a problem of stabilization of the system? Should I run
a
> deeper minimization, or a longer NVT+NPT in my system?
> I'm quite puzzled especially because the system is the same in all cases,
> the only thing I changed in my simulations is the gen_seed.
> Could anybody give me some suggestions about it?
> Thank you very much
> Anna
>
>
>
>
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-- 
Tsjerk A. Wassenaar, Ph.D.

post-doctoral researcher
Molecular Dynamics Group
* Groningen Institute for Biomolecular Research and Biotechnology
* Zernike Institute for Advanced Materials
University of Groningen
The Netherlands

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