fix npt problem: volume expended too much

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1

Dear Lammps

I am performing the system of two parallel SiO2 walls are immersed in water. The system can goes well with NVT. (without minimize energy)
After NVT equilibrium, I use NPT for output properties. However, the volume of the system expended with time step. A very big empty hole formed after running NPT 100ps. Also, the two walls moved away from each other after NPT, although I used fix rigid on them.

Does anyone know why it happens like this? Below are the calculation for NVT and NPT.

NVT part

fix 1 water shake 0.0001 20 0 b 1 a 1
fix 2 water nvt temp $T $T 200.0
fix 3 wallup rigid/nvt single temp $T $T 200.0 force 1 off off off torque 1 off off off
fix 4 walllow rigid/nvt single temp $T $T 200.0 force 1 off off off torque 1 off off off

##NPT part

fix 1 water shake 0.0001 20 0 b 1 a 1
fix 2 water npt temp $T $T 100.0 z 1.0 1.0 100.0 dilate all
fix 3 wallup rigid/nvt single temp $T $T 100.0 force 1 off off off torque 1 off off off
fix 4 walllow rigid/nvt single temp $T $T 100.0 force 1 off off off torque 1 off off off

Thank you very much in advance.

Liyi

2

Have you monitored the pressure of your system as you apply NPT,
as compared to your target (requested) pressure. The difference between
those is what drives the volume to expand or contract.

Steve