GROMACS version: 2022.2

GROMACS modification: No

I simulate molecular dynamics of 1024 water. Water molecule is made by quantum calculation using Orca with B3LYP functional and def2-TZVP basis-set. I set GAFF by AmberTools. The simulation consists of minimization; NVE, NVT (310 K), and NPT (1 bar) equilibration; and NVE production.

The results seems good for minimization, NVE and NVT equilibration. Potential energy is converged and temperature is steady. But, during NPT equilibration, the system blow-up from 31.3 nm^3 to 38.5 nm^3. It affects the density of the system. The density should be 0.99 g/cm^3, but the result is 0.80 g/cm^3.

I’ve tried using different barostat: Berendsen (tau-p = 0.1) and C-rescale (tau-p = 2 and 4). The volume and density doesn’t give any difference, but the pressure is. Pressure for Berendsen is 1.44 bar (close to 1) and for C-rescale is 6.53 fot tau-p = 2 and 10.30 for tau-p = 4.

Can anyone help me how to overcome this problem?