searching for other particles in Lagrangian frame
 

Dear all,
I need a search algorithm inside my code to find the interacting force between two arbitrary particles. Better to say, I want to find the force of particle i on particle j (they should not necessarily be in contact). I know that I must use forAll ... for any individual particle. However, I have some problems in writing the code. Does anybody have experience in this field. Please help! :(

Best wishes,

Rjam