How to fix negative pressure?

GROMACS version 2021:
GROMACS modification: No
My name is Matteo and I’m currently learning GROMACS so I have no experience at all.
After NVT equilibration I have always a negative pressure (-300 bar).
I want to have a positive pressure of about 560 bar without using barostat but only varying box size.
Can someone help me ?
I’m using this nvt script.

You have to equilibrate the system under the desired conditions and ensemble, so in your case if you want an NVT ensemble but with fixed pressure, you have to equilibrate under NPT at that pressure and once you achieve it, switch back to NVT.

Ok thank you.
So the first equilibration is NPT and the second is NVT ?

The NVT phase wouldn’t necessarily be considered equilibration, but yes, the order I’m suggesting is NPT then NVT.

Ok thank you.