I am using lammps-9Dec14. I am simulating methane molecules (modelled as single site LJ molecules) confined in a graphene slit-like channel of width 7.62nm at 300K. I want to calculate the local pressure profiles (normal and lateral) along the z direction (perpendicular to the graphene walls). I did this with aid of compute peratom command in Lammps and dump the results using following commands:
compute peratom methane stress/atom methT
dump 2 methane custom 500 dump_diag_stress.dat id type x y z fz c_peratom c_peratom c_peratom
I post-proccessed the dump file results in slit bins of width 0.1 nm with x and y dimensions parallel to the graphene walls. For normal pressure, for example, I averaged the -1c_peratoms and divide it by the the volume of each bin. However, the normal pressure turned out to be z-dependent which should be a constant value due to the mechanical equilibrium in z direction. And lateral pressure (0.5(pxx+pyy)) profile is half of what it should be. I looked into the pressure formulation of Lammps and it seems there is no error. I would appreciate it if anyone can help me figure out what’s wrong with pressure profiles from Lammps ( especially normal pressure which is not a constant)
I have attached the plots to this post comparing the pressure profiles to that of Gromacs software.
P.S. density profile of methane molecules are correct, which I guess there is no serious problem with the simulation.
Mohammad Hossein Motevaselian
3216 Beckman Institute
405 North Mathews Avenue
Urbana, Illinois 61801
2Pl_lammps.eps (47.4 KB)
Pl_lammps.eps (59.9 KB)
Pn_lammps.eps (41.2 KB)