Contact Area Particle-Walls Cohesive model

Hi everyone,

I would like to know how LIGGGHTS calculates the contact area between Particle-WALL in a cohesive SJKR model. The given formula is:

A = Pi/4 * ((dist-Ri-Rj)*(dist+Ri-Rj)*(dist-Ri+Rj)*(dist+Ri+Rj) )/(dist*dist)

how are the values of 'dist' and radii evaluated for particle-wall interactions?

Thank you in advance for your kind reply,

John