# Gromacs freeze proplems
**URL:** https://gromacs.bioexcel.eu/t/gromacs-freeze-proplems/4047
**Category:** User discussions
**Tags:** mdrun, restraints
**Created:** [April 20, 2022, 1:22pm UTC](https://gromacs.bioexcel.eu/t/gromacs-freeze-proplems/4047 "2022-04-20T13:22:16Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Hsin](https://gromacs.bioexcel.eu/u/Hsin)
#### Post date: [April 20, 2022, 1:22pm UTC](https://gromacs.bioexcel.eu/t/gromacs-freeze-proplems/4047/1 "2022-04-20T13:22:16Z")
GROMACS version:VERSION 4.5.5
GROMACS modification: No
Hi Gromacs users,
I am having a fatal error with the freeze groups function. What I want to do is make a water molecule (atom 145-148 ) free to move, and freeze all the rest.
in my .ndx file
[fzgp1]
1 2 3 …
16 …144
[fzgp2]
149 …
…31168
[Ion]
31169 …
…31568
in my .mdp file
…
freezegrps = fzgp1 fzgp2 Ion
freezedim = Y Y Y Y Y Y Y Y Y
in my .top file
#include “/home/b06504051/hsin/hsin/7792200200/ffnonbonded.itp”
#include “atomtypes.itp”
#include “tip4p\_Ice.itp”
#include “Na.itp”
#include “Cl.itp”
#include “frz.ndx”
[system]
7792200200
[molecules]
H2O 7792
Na 200
Cl 200
After running the following command
gmx grompp -f 1.min.mdp -c naclsolution.g96 -r naclsoluiton.g96 -n frz.ndx -p topol.top -po 1.minout.mdp -o 1.min.tpr
and it shows a fatal error
ERROR 1 [file index\_0420\_1.ndx, line 1]:
Invalid directive fzgp1
I am not sure where the problem is. Do anyone have any suggestions?
---
### Author:  [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)
#### Post date: [April 20, 2022, 2:09pm UTC](https://gromacs.bioexcel.eu/t/gromacs-freeze-proplems/4047/2 "2022-04-20T14:09:25Z")
Hi,
Removing the inclusion of index file (#include “frz.ndx”) from the top file should solve the error. Index files are input with the option -n to gmx grompp
\Alessandra
---
### Author:  [@Hsin](https://gromacs.bioexcel.eu/u/Hsin)
#### Post date: [April 20, 2022, 2:37pm UTC](https://gromacs.bioexcel.eu/t/gromacs-freeze-proplems/4047/3 "2022-04-20T14:37:06Z")
It works!!!
Thank you very much.