# Simulation box dimensions precisely

Hi all,
I want to simulate a liquid between to parallel walls (Couette flow) like the one in the LAMMPS example directory but in 3d. The problem is that I use lattice fcc but since that sets atoms on a lattice I can’t get whatever wall thicknesses that I want to set and it should a multiple of a lattice side size. E.g. if the lj reduced density is set 0.755 then lattice sides size are aound 1.74 (lj reduced units) and I want the walls to have a thickness of say exactly 4.35 but I can’t create such a thickness as it’s not a multiple of lattice side size (1.74)

I was thinking by using random option in create_atoms I can create regions exactly the sizes that I need and throw atoms in the there but as I freeze the walls during the simulation (fix setforce) I don’t know how to do energy minimization and equlibrate the problem.

Here is how I set up the problem

region total block -10 10 0 68.7 -10 10 units box
region wall1 block -10 10 0 4.35 -10 10 units box
region phaseI block -10 10 4.35 64.35 -10 10 units box
region wall2 block -10 10 64.35 68.7 -10 10 units box
create_box 3 total

You can use units lattice (the default) when you define
regions, and then the length scale is in lattice spacings.

Steve

Yes, I have tried that option but I have two question about that:

1. Can I create regions that are not a integer multiple of the lattice size? like for the example I provided if I set the region height for wall thickness as ylo=0 and ymax= 2.5 (which is 4.35/1.74) then does that create that half a lattice such that the surface of wall is on 4.35 such that no fluid molecules can go below that?

2. I tried to set up a problem by having the simulation box size as exact multiple of the lattice sides as :

lattice fcc 0.755

yes, you can use fractional values - all the "units" option
sets is a mutlipier, i.e. a scale factor.

Steve