fixed rod p2p typo

cpp/particles/p2p/p2p_ghost_collisions.hpp

@@ -218,7 +218,7 @@ public:
                              real_number /*rhs_part2*/[],
                              const real_number dist_pow2,
                              const real_number /*cutoff*/,
-                             const real_number xseparation,
+                             const real_number xseparation, /* This separation is x1-x2 */
                              const real_number yseparation,
                              const real_number zseparation){
         switch(this->current_particle_shape)
@@ -250,8 +250,8 @@ public:
                         xq = x * pos_part2[IDXC_X+3] + y * pos_part2[IDXC_Y+3] + z * pos_part2[IDXC_Z+3];
                         /* t and s parametrize the two rods. Find min distance: */
                         assert(this->cylinder_length > 0);
-                        t = 2.0/(this->cylinder_length*(pq*pq-1.0))*(-xp+pq*xq);
-                        s = 2.0/(this->cylinder_length*(pq*pq-1.0))*(-pq*xp+xq);
+                        t = 2.0/(this->cylinder_length*(pq*pq-1.0))*(xp-pq*xq);
+                        s = 2.0/(this->cylinder_length*(pq*pq-1.0))*(pq*xp-xq);
                         /* Test if -1<s<1 and -1<t<1 */
                         if( abs(t)<=1.0 and abs(s)<=1.0 )
                         {