Hi LAMMPS users,
I used fix press/berendsen to control external pressure and fix langevin to control temperature, fix nve for time integration in my polymer network system. Before tensile test, four equilibration stage have been applied to relax the system. First, use fix langevin and fix nve at 600 K to relax system, second, make use of fix press/berendsen to keep external pressure 0, fix langevin and fix nve at 600K to relax the system again. Third, cooling the system to 300K in the same commands of the second stage. At last, I relaxed the system at 300K enough time until volume and density are both unchanged.
After equilibration stage, I used fix deform to change the box size in x direction to obtain tensile stress and strain, the external pressure set at 0 in y and z direction and the temperature kept at 300 K. The commands format are shown as follows:
fix 1 all nve
fix 2 all press/berendsen y 0.0 0.0 {pdamp} z 0.0 0.0 {pdamp}
fix 3 all langevin {T} {T} ${tdamp} 231569 scale 2 1.3182
fix 4 all deform 1 x erate 1e-4 units box remap v
thermo 1000
thermo_style custom step temp ke pe etotal epair ebond eangle lx ly lz vol pxx pyy pzz density
timestep ${dt}
Thermodynamic output at tensile stage:
Timestep pxx pyy pzz density
10000 -0.40152 -0.40152 0.01826 0.32429
20000 0.20781 0.20781 -0.09526 0.32341
30000 0.01763 0.01763 -0.17381 0.32268
40000 0.21611 0.21611 -0.05325 0.32176
50000 0.08923 0.08923 -0.19923 0.32102
But the either thermodynamic output pxx or the compute pressure commands output pxx are always both around 0. What was strange, the thermodynamic output value of pxx is the same as pyy. Someone can give me some suggestions to get the right stress? Thank you very much!