Writing a fix to apply a 2D spring force

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

• The new files that are created are fix_addforceXY.cpp and fix_addforceXY.h.
• The existing files that have been modified are fix_addforce.cpp and fix_addforce.h.
• The methods that are modified are constructor, init(), and post_force().
• The input parameters are x0, y0, kx, and ky.
• Other quantities that are introduced are kxstr[], kystr[], kxstyle, kystyle, kxvar, kyvar, and ework[].
• The LAMMPS syntax is fix FIX_NAME GROUP addforceXY x0 y0 zvalue kx ky (we will look at this syntax shortly in this section).

In this section, we will write a fix that applies a 2D, non-radially symmetric spring force on an atom directed toward a specified point.

Theory (2D spring force)

In this setup, we have an atom in a 2D system at location (x, y) that experiences a force pointing toward a point that depends on the spring constants and in the x and y directions...