Binary gas adsorption + NPT simulation

Dear all

I am trying to adsorb a mixture of CO2-CH4 on nanotube (10-10) using NPT simulation, for this purpose I’m using LAMMPS (8 Feb 2019). At first, I simulated a mixture of gases without nanotube and the bulk density was correct. After that, I examined the adsorption of methane and carbon dioxide purely on the nanotube and the results were consistent with the results of previous work. My problem appears when I have a mixture of two gas and nanotube (5% CO2 and 95% CH4). The results are fine until the pressure increasing to 15 bar or higher. After 1ns or more, the bulk density jumps higher than expected. I haven’t this problem with the equal mixture! Any suggestion?

(I attached my files below.)

cnt10.rar (615 KB)