[gmx-users] Add gromacs forcefield w/ virtual site

[Marcelo Silva]

Thank you Chris for your answer:

1) The molecule has no net charge because the virtual site in the center 
of mass is a point charge twice the charge in the O atom.
2) Until now I've created 5 files but I don't know if I am doing the 
right thing:

    *   forcefield.itp

#define _FF_OXY

[ defaults ]
; nbfunc        comb-rule         gen-pairs        fudgeLJ     fudgeQQ
1                    2                        no                    
1.0            1.0

#include "ffnonbonded.itp"
#include "ffbonded.itp"

    *    ffbonded.itp

[ bondtypes ]
; i    j       func
   O    O         5  (I've used function 5 because the forcefield has a 
fixed length, is it right?) ;

    *    ffnonbonded.itp

[ atomtypes ]
; name  at.num        mass              charge   ptype              
sigma          epsilon
      O                8       15.99940        -0.123       A         
3.01300e-01    4.0780822e-01
    COM            0       0.000000         0.246       V         
0.00000e+00    0.0000000e+00

    *   atomtypes.atp

O     15.99940  ; Oxygen Atom
COM   0.0       ; Virtual site (COM charge)

    *   residues.rtp

; Oxygen
[ OXY ]
  [ atoms ]
      O         O              -0.123    1
      COM   COM        0.246    1
  [ bonds ]
     O    O

3) Is it necessary more or less files? And in which file do I 
specificate de virtual site?
4) The virtual site in the COM is [ virtual_sites2 ] with a = 0.5 or [ 
virtual_sitesn ]?

Thank you again for your time.
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