# fix wall/lj93: Define the wall position

Dear Lammps users,

I want to define a rough wall by using wall/lj93 command including a sinusoidal function for the wall (ZRough) as a function of x coordinate like:

variable ZRough equal “sin(2PIv_x)”

But I couldn’t find anything in the documentation and mailing list showing how to define a position variable for x direction to use as “v_x” in the above formula. Moreover, my wall/lj93 command line is:

fix walllo all wall/lj93 zlo \${ZRough} 0.42 1.122 50.0 units box

I was wondering if somebody could help me with this issue?

Mey

Dear Lammps users,

I want to define a rough wall by using wall/lj93 command including a
sinusoidal function for the wall (ZRough) as a function of x coordinate
like:

variable ZRough equal "sin(2*PI*v_x)"

But I couldn't find anything in the documentation and mailing list showing
how to define a position variable for x direction to use as "v_x" in the
above formula. Moreover, my wall/lj93 command line is:

fix walllo all wall/lj93 zlo \${ZRough} 0.42 1.122 50.0 units box

I was wondering if somebody could help me with this issue?

if the documentation does say that you can do it, then you cannot do it.
if you want a wall that is not flat, you will have to write a new
fix.style or approximate it with immobile particles.

axel.

So isn’t there a way to define the global x coordinate?

Mey

So isn't there a way to define the global x coordinate?

which part of "if you want a wall that is not flat, you will have to
write a new
fix.style or approximate it with immobile particles" did you not understand?

axel.

There are several problems with your conceptual

model of an LJ 9/3 wall that is sinusoidal.

1. the 9/3 formula comes from integrating over

a flat half-plane. If it’s wavy, the interactions are

no longer 9/3

1. To have a wall/particle interaction, you need

to be able to define and compute a distance between

a particle and the wall. There is no simple or even

unique definition of distance to a wavy surface.

A particle could be close to multiple locations on the

wall. If you only use the closest point on the wall,

it could change dramaticaly for an epsilon change

in the particle postiion, leading to non-continuous forces.

What most people do if they want a rough surface

is build the wall out of particles.

Steve