Symmetry boundary conditions

Please post to the list, not to me.

If you are using fix wall/reflect, I don’t see

how it could alter the density profile at the wall.

Maybe it has to do with how you are binning

your atoms to compute the profile.


Dear Steve,
Thanks a lot for your answer. But it seems that I should look for another reason. I use bins with a thickness of 0.5 Angstrom in a way that the boundary of the last one locates on the reflecting wall. But the reduction in the density profile starts at a distance of about 6 Angstrom from the reflecting wall and continues until the fluid density becomes zero on the wall surface. The consequent is that the fluid density increases far away from the wall.
My fluid is SPC water containing cations and anions which are under the coulomb forces in addition to the lennard jones interactions. I use pppm algorithm with a slab correction in order to take into account the coulomb long range forces.
I think that the problem is that the coulomb forces have been made weaken near the reflecting wall which causes a drop in the density profile. Is it true? If Yes, how can I solve this problem? And if no, what is the cause of this reduction in fluid density?

I don’t know what would be causing the problem. Stan can comment

on PPPM near a wall. You could try running your model w/out PPPM

of charges to see if the density smooths out. Or run a simple LJ model

with a reflecting wall and verify that your binning procedure is not

producing odd density profiles.


It seems to me that if the system is a non-equilibrium system or one that has not reached equilibrium, you would get this kind of behavior. One boundary is a reflective wall and the other is a moving/shearing boundary. I don’t think there is any reason for the atoms to distribute themselves homogeneously. Something to think about.