/usr/share/gromacs/top/charmm27.ff/tip4p.itp is in gromacs-data 2018.1-1.
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; Note the strange order of atoms to make it faster in gromacs.
;
[ moleculetype ]
; molname nrexcl
SOL 2
[ atoms ]
; id at type res nr residu name at name cg nr charge
1 OWT4 1 SOL OW 1 0.0
2 HWT4 1 SOL HW1 1 0.52
3 HWT4 1 SOL HW2 1 0.52
4 MWT4 1 SOL MW 1 -1.04
#ifndef FLEXIBLE
[ settles ]
; OW funct doh dhh
1 1 0.09572 0.15139
#else
[ bonds ]
; i j funct length force.c.
1 2 1 0.09572 502416.0 0.09572 502416.0
1 3 1 0.09572 502416.0 0.09572 502416.0
[ angles ]
; i j k funct angle force.c.
2 1 3 1 104.52 628.02 104.52 628.02
#endif
[ exclusions ]
1 2 3 4
2 1 3 4
3 1 2 4
4 1 2 3
; The position of the virtual site is computed as follows:
;
; O
;
; D
;
; H H
;
; const = distance (OD) / [ cos (angle(DOH)) * distance (OH) ]
; 0.015 nm / [ cos (52.26 deg) * 0.09572 nm ]
; Vsite pos x4 = x1 + a*(x2-x1) + b*(x3-x1)
[ virtual_sites3 ]
; Vsite from funct a b
4 1 2 3 1 0.128012065 0.128012065
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