[gmx-users] problem with adding a carbonate molecule to .rtp file

[balamurugan r]

  Sir,
I have a protein transferrinthat contains a Fe and a CO3 hetero molecule. I used the ffG43a1 force field. When i ran pdb2gmx i got an ERROR saying the CO3 residue was not found in the database. I tried to add the molecule to the datadase i got the itp file  from PRODRG2 server :

;       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
CO3      3

[ atoms ]
;   nr      type  resnr resid  atom  cgnr   charge     mass
     1        OM     1  CO3     OAA     1   -0.221  15.9994
     2         C     1  CO3     CAC     1    0.494  12.0110
     3        OM     1  CO3     OAB     1   -0.221  15.9994
     4        OM     1  CO3     OAD     1   -0.052  15.9994

[ bonds ]
; ai  aj  fu    c0, c1, ...
   1   2   1    0.125    418400.0    0.125    418400.0 ;   OAA  CAC
   2   3   1    0.125    418400.0    0.125    418400.0 ;   CAC  OAB
   2   4   1    0.125    418400.0    0.125    418400.0 ;   CAC  OAD

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

[ angles ]
; ai  aj  ak  fu    c0, c1, ...
   1   2   3   1    126.0       502.1    126.0       502.1 ;   OAA  CAC  OAB
   1   2   4   1    126.0       502.1    126.0       502.1 ;   OAA  CAC  OAD
   3   2   4   1    126.0       502.1    126.0       502.1 ;   OAB  CAC  OAD

[ dihedrals ]
; ai  aj  ak  al  fu    c0, c1, m, ...
   2   4   3   1   2      0.0 1673.6 0      0.0 1673.6 0 ; imp   CAC  OAD  OAB  OAA

and then i tried to update the ff<***>.rtp file by adding the following lines to the last of the .rtp file 
[CO3]
[atoms]
OAA OM  -0.221  0
CAC  C  0.494   1
OAB  OM  -0.221 0
OAD  OM  -0.052 2
  I obtained the details from the .itp file from PRODRG2 server
but still i am not able to run pdb2gmx the error is that :
  "Atom C in residue CO3 330 not found in rtp entry with 4 atoms while sorting atoms "

  I dont know how to proceed further. 
please guide me where i went wrong
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