/usr/share/gromacs/top/amberGS.ff/tip4pew.itp is in gromacs-data 2018.1-1.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 | [ moleculetype ]
; molname nrexcl
SOL 2
[ atoms ]
; id at type res nr res name at name cg nr charge mass
1 OW_tip4pew 1 SOL OW 1 0 16.00000
2 HW_tip4pew 1 SOL HW1 1 0.52422 1.00800
3 HW_tip4pew 1 SOL HW2 1 0.52422 1.00800
4 MW 1 SOL MW 1 -1.04844 0.00000
#ifndef FLEXIBLE
[ settles ]
; i 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
[ virtual_sites3 ]
; Vsite from funct a b
4 1 2 3 1 0.106676721 0.106676721
[ 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
;
; V
;
; H H
;
; Ewald tip4p:
; const = distance (OV) / [ cos (angle(VOH)) * distance (OH) ]
; 0.0125 nm / [ cos (52.26 deg) * 0.09572 nm ]
; then a = b = 0.5 * const = 0.106676721
;
; Vsite pos x4 = x1 + a*(x2-x1) + b*(x3-x1)
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