Analyzing the Fix Wall and Fix Wall/LJ126 classes
In molecular dynamics, a wall serves as a boundary condition that exerts a force on an atom that approaches its interaction range. Unlike forces exerted by typical atoms, the wall force is not radially symmetric and is usually perpendicular to the wall. The Fix Wall class creates such planar walls that are perpendicular to the x, y, or z axis and can exert forces according to pre-defined potential functions in its child classes.
In a LAMMPS input script, the corresponding syntax for implementing a wall is as follows:
fix FIX_ID GROUP_ID WALL_STYLE FACE COORDS …
The parameters described in the manual (https://lammps.sandia.gov/doc/fix_wall.html) include WALL_STYLE, which is the type of potential between the wall and an atom, such as Lennard-Jones (LJ), Morse, and Harmonic; FACE, which is the direction that the wall is facing in, and can be one of 
; and...
