Gas pahse simulation in gromacs

GROMACS version:2019
GROMACS modification: No


I want to simulation of carbon dioxide (Co2) with gromacs in gas phase state, but after NPT run, I get wrong mass density near 1100kg/m3 (real density 1.7 kg/m3) why?
I used values for compressibility default, 4.5e-5.
I calculated mass density with gmx energy.
force field: opls and amber (check two forms)


As I sent to you in your ResearchGate message:

Using liquid water compressibility with a barostat will force your system into an unphysical liquid state. Simulations of gases should be done using an NVT ensemble. Please ask all future GROMACS questions via the GROMACS users forum.