[gmx-users] need suggestion for building TMA (tetramethylammonium) topology and using as a general ion

[Simple]

   In chemical experiments that ion effect is important,tetramethylammonium 
(TMA) is usually added to compensate the ion strength.I would like to test it 
in simulation. There were experiments showed that the ion contributed a lot 
to the distinct results.
   I looked around but not found a topology  of TMA is ready-to-use. So I 
decided to create one. Thanks for  Schuettelkopf and  van Aalten, The PRODRG 
server do all the things. Since I am still a newbie for Gromacs and not very 
confident with my chemistry knowledge,I wonder if the parameters in the 
topology is rational? (The topology is present at the last.)

   The second problem is how can I use this topology as a general ion. 
   It is difficult to add them into gro file by hand because I should put a 
lot of TMA into the solutions.
   If I can treat it as a general ion, such as Na or Mg,etc, which will save 
me a lot. My straight forward thought is to add the information into the 
ion.itp, so genion can find and use it. If this work, I believe others would 
also benefit from it, since TMA is so widely used. Does it work well?
   If this method is not so good,I think I can write a simple program to 
substitute the ion in the gro with TMA. Of course, I would rewrite the 
position of all the TMA atom and renumber the atom. For the top file, I need 
only replace the ion name with TMA.
   What should I pay a lot of attention to for these method?

   If something I wrote are totally wrong, please not hesitate to let me know. 
Thanks.

topology created by PRODRG 
;       
;       This file was generated by PRODRG version 051202.0518
;       PRODRG written/copyrighted by Daan van Aalten
;       
;       Questions/comments to dava at davapc1.bioch.dundee.ac.uk
;       
;       When using this software in a publication, cite:
;       A. W. Schuettelkopf and D. M. F. van Aalten (2004).
;       PRODRG - a tool for high-throughput crystallography
;       of protein-ligand complexes.
;       Acta Crystallogr. D60, 1355--1363.
;       
;       

[ moleculetype ]
; Name nrexcl
TMA      3

[ atoms ]
;   nr      type  resnr resid  atom  cgnr   charge     mass
     1       CH3     1  TMA      C1     1    0.076  15.0350   
     2        NL     1  TMA      N1     1    0.696  14.0067   
     3       CH3     1  TMA      C3     1    0.076  15.0350   
     4       CH3     1  TMA      C4     1    0.076  15.0350   
     5       CH3     1  TMA      C2     1    0.076  15.0350   

[ bonds ]
; ai  aj  fu    c0, c1, ...
   1   2   1    0.147    376560.0    0.147    376560.0 ;    C1   N1   
   2   3   1    0.147    376560.0    0.147    376560.0 ;    N1   C3   
   2   4   1    0.147    376560.0    0.147    376560.0 ;    N1   C4   
   2   5   1    0.147    376560.0    0.147    376560.0 ;    N1   C2   

[ pairs ]
; ai  aj  fu    c0, c1, ...

[ angles ]
; ai  aj  ak  fu    c0, c1, ...
   1   2   3   1    109.5       376.6    109.5       376.6 ;    C1   N1   C3   
   1   2   4   1    109.5       376.6    109.5       376.6 ;    C1   N1   C4   
   1   2   5   1    109.5       376.6    109.5       376.6 ;    C1   N1   C2   
   3   2   4   1    109.5       376.6    109.5       376.6 ;    C3   N1   C4   
   3   2   5   1    109.5       376.6    109.5       376.6 ;    C3   N1   C2   
   4   2   5   1    109.5       376.6    109.5       376.6 ;    C4   N1   C2   

[ dihedrals ]
; ai  aj  ak  al  fu    c0, c1, m, ...
   2   1   3   4   2     35.3  836.8       35.3  836.8   ; imp    N1   C1   C3   
C4