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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