Volume fluctuation widely during NPT ensemble (water-methanol mixture)

Dear LAMMPS users

I am using LAMMPS to study water-methanol mixture.

I am trying to use NPT to equilibrate the system, and I find something not normal.

The system consists 600 Tip4p/2005 water and 85 methanol molecules.

If I use NPT with isotropic barostat (Nose-Hoover), the system is fine.

But when I use anisotropic barostat (Nose-Hoover), the volume fluctuates very widely.

I played with drag factor, relaxation time, and reduced time step, but they are not solving the widely fluctuation of volume.

I did the same run for pure methanol and water, and isotrobic and anisotropic barostat deliver similar results.

I am wondering is this because the dipole moment of methanol and water molecule is not the same?

I attach my input file for you guys to check


Tzu-Jen Lin

in.tip4p-300k-pre (2.46 KB)

Tip4p_MeOH_20wt.data (247 KB)