“…Please note that while r i,j = r j,i , this symmetry does not hold for dx or dy (meaning that dx i,j = dx i,j ) as either of these is directional, i.e., will always differ in their sign when reversed (dx i,j = −dx i,j ). This comes back in Equations ( 5)- (7) where it determines the resulting direction of the force, i.e., whether the force is attractive or repulsive.…”