# How to restrain to some specified waters for OPC model **URL:** \ **Category:** User discussions\ **Tags:** restraints\ **Created:** [December 11, 2023, 1:24am UTC](https://gromacs.bioexcel.eu/t/how-to-restrain-to-some-specified-waters-for-opc-model/7836 "2023-12-11T01:24:01Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![Shin](https://avatars.discourse-cdn.com/v4/letter/s/54ee81/32.png) [@Shin](https://gromacs.bioexcel.eu/u/Shin)\ **Post date:** [December 11, 2023, 1:24am UTC](https://gromacs.bioexcel.eu/t/how-to-restrain-to-some-specified-waters-for-opc-model/7836/1 "2023-12-11T01:24:01Z")
GROMACS version:2021.6 GROMACS modification: No Here post your question Dear Gromacs users I have the same problem as in [How to treat specific water molecules as ligand?](https://gromacs.bioexcel.eu/t/how-to-treat-specific-water-molecules-as-ligand/3470). I’m setting .top file for MD simulation with OPC model for water. I want to use positional constraints on the three waters(name: FIX) that coordinate to the sodium ion. Using the above link’s answer, I have rewritten the [settles] section. Are there any other sections to be rewritten? ```auto [moleculetype] ; Name nrexcl FIX 3 [atoms] ; nr type resnr residue atom cgnr charge mass typeB chargeB massB ; residue 1 FIX rtp FIX q 0.0 1 OW 1 FIX O 1 0.00000000 16.000000 ; qtot 0.000000 2 HW 1 FIX H1 2 0.67914200 1.008000 ; qtot 0.679142 3 HW 1 FIX H2 3 0.67914200 1.008000 ; qtot 1.358284 4 EP 1 FIX EPW 4 -1.35828400 0.000000 ; qtot 0.000000 #ifdef FLEXIBLE [bonds] ; ai aj funct c0 c1 c2 c3 2 1 1 0.08724 462750.400000 3 2 1 0.13712 462750.400000 3 1 1 0.08724 462750.400000 #else [constraints] 1 2 1 0.08724331 1 3 1 0.08724331 2 3 1 0.13712051 #endif [virtual_sites3] ; Site from funct 4 1 2 3 1 0.147721 0.147721 [exclusions] 1 2 3 4 2 1 3 4 3 1 2 4 4 1 2 3 #ifdef REST_ON #include "/home/itp/posre_FIX.itp" #endif ```