[gmx-users] numbering of .gro file

[Mark Abraham]

On 13/05/11, Anna Marabotti  <anna.marabotti at isa.cnr.it> wrote:
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> Dear 
> gmx-users,
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> I'm simulating a 
> homodimeric protein obtained by homology modelling. In the .pdb file, I have 2 
> chain identifiers (A and B). I used Gromacs 4.0.7 to prepare the .gro files for 
> simulation (up to nvt); then I was forced to switch to Gromacs 4.5.4 on another 
> machine to continue with npt and full MD. I used the files from Gromacs 4.0.7 to 
> continue, and it seems to me that all went OK. Now I have the files from the 
> full MD, and I started to analyze them.
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> I used g_rmsf to 
> analyze the fluctuations of the residues, and I found that the results for the 
> two subunits of the protein are superimposed. In the file .gro that comes from 
> the MD as well as in the file .gro that comes from the npt MD made with Gromacs 
> 4.5.4, the number of residues of the second subunit does not continue after the 
> first subunit (i.e. after the last 359th residue of the first subunit, I see 
> that the first residue of the second subunit is numbered 1 instead of 360), but 
> in the nvt file produced by Gromacs 4.0.7 that I provided to Gromacs 
> 4.5.4, the numbering of the two subunits is 
> sequential. 
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> My questions 
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> 1) why Gromacs 4.5.4 
> changed the numbering of the residues in the two subunits, even if it started 
> from a .gro file?
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The trajectory files do not number the atoms, so the only way you can generate such numbering is by matching them with a structure file that also has such numbering. It's hard to be any more specific in the absence of command lines. If you just match the trajectory file with a more suitably numbered structure file (with atom ordering preserved) then everything is fine.


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> 2) Does this affect 
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I highly doubt it.


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> 3) how can I 
> separate, or renumber, the resulting .gro files in order to have a graph in 
> which the fluctuations are not superimposed?
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I rather expect that the input file to your NPT grompp had this "feature". In any case, you need to identify where it first arose in your workflow in order to work out why it happened and what to do about it.

Mark

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