# Running GROMOS54A7 with TIP4P water
**URL:** https://gromacs.bioexcel.eu/t/running-gromos54a7-with-tip4p-water/1976
**Category:** User discussions
**Created:** [April 16, 2021, 6:13pm UTC](https://gromacs.bioexcel.eu/t/running-gromos54a7-with-tip4p-water/1976 "2021-04-16T18:13:42Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@megamence](https://gromacs.bioexcel.eu/u/megamence)
#### Post date: [April 16, 2021, 6:13pm UTC](https://gromacs.bioexcel.eu/t/running-gromos54a7-with-tip4p-water/1976/1 "2021-04-16T18:13:43Z")
GROMACS version: 2020.5
GROMACS modification: Yes
Hello,
I am trying to run a simulation of the thermoresponsive hydrogel Poly(N-isopropylacrylamide), as per the paper DOI: [10.1016/j.fluid.2010.03.025](http://dx.doi.org/10.1016/j.fluid.2010.03.025)
In the paper, on page 20, they mention that they used GROMOS with TIP4P water to run their simulation. I have been trying to run this simulation, but I am facing quite a few hurdles. I know that TIP4P is _not_ recommended with GROMOS, but I want to figure out how to run it, as a learning exercise.
Initially, I was getting the error, atomtype OWT4 not found, so I added it to atomtypes, and Tten, I added the TIP4P atomtypes in the GROMOS54A7\_atb.ff/ffnonbonded.itp for tip4p water from CHARMM27.ff/ffnonbonded.itp.
I obtained the .itp and .pdb file for my polymer from ATB.
However, once I did that, I started getting the following error:
ERROR 1 [file topol.top, line 20]:
atom H29 (Res XCN0-1) has mass 0 (state A) / 0 (state B)
And I have 572 (!) of such errors, where
atom XXX (Res XCN0-1) has mass 0 (state A) / 0 (state B)
Is there a way to fix this error? How did the people who wrote that paper manage to pull this off?
I appreciate any advice you have for me.
---
### Author:  [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)
#### Post date: [April 19, 2021, 1:03pm UTC](https://gromacs.bioexcel.eu/t/running-gromos54a7-with-tip4p-water/1976/2 "2021-04-19T13:03:44Z")
> [@megamence](#):
>
> Initially, I was getting the error, atomtype OWT4 not found, so I added it to atomtypes, and Tten, I added the TIP4P atomtypes in the GROMOS54A7\_atb.ff/ffnonbonded.itp for tip4p water from CHARMM27.ff/ffnonbonded.itp.
You will need to transform the values from σ/ε to C6/C12 since these force fields use different conventions for the LJ expression.
> [@megamence](#):
>
> However, once I did that, I started getting the following error:
Does execution of `grompp` work with unmodified force field files? The problem seems unrelated to water.
> [@megamence](#):
>
> How did the people who wrote that paper manage to pull this off?
Write to the corresponding author and ask :)