Writing a fix to apply a custom wall force

Here is a list of changes that are made to the source code:

• The existing files that have been modified are fix_wall_region.cpp and fix_wall_region.h.
• The methods that have been modified are constructor, init(), and post_force().
• The new method added is lj126Expanded().
• The new input parameter introduced is deltaLJ.
• The LAMMPS syntax is fix FIX_NAME GROUP wall/region REGION lj126Expanded EPSILON SIGMA DELTALJ CUTOFF (we'll review this syntax in this section).

In this section, we will add a custom wall potential to the inventory of existing ones in the Fix Wall/Region class.

Theory (expanded Lennard–Jones wall force)

The Fix Wall/Region class (https://lammps.sandia.gov/doc/fix_wall_region.html) defines the surface of a region as a wall that can apply forces to an atom that interacts with it. This force of interaction can be defined in terms of a potential function (for example, Lennard–...