Problems With Vector Reflection with Particle Collisions

I was wondering whether I made a math mistake in my particle collision simulation found here. The particles don't seem to separate properly during collision resolution. Here is a code snippet from the function which separates particles and changes their velocities:

                    //particle 1
                var pi = particles[i];
                //particle 2
                var pj = particles[j];

                //particle 1 to particle 2
                var pimpj = pi.mesh.position.clone().sub(pj.mesh.position); 
                //particle 2 to particle 1
                var pjmpi = pj.mesh.position.clone().sub(pi.mesh.position); 
                //if colliding (radius is 20)
                if(pimpj.length() < 20 && pimpj.length() != 0) 
                {


                    //reflect velocity off of 1->2
                    pi.velocity = pi.velocity.reflect(pimpj.clone().normalize()).multiplyScalar(restitution);
                    //reflect velocity off of 2->1 
                    pj.velocity = pj.velocity.reflect(pjmpi.clone().normalize()).multiplyScalar(restitution);

                    //move particle 1 to appropiate location based off of distance in between
                    var pip = pi.velocity.clone().normalize().multiplyScalar(20-pimpj.length());
                    //move particle 2 
                    var pjp = pj.velocity.clone().normalize().multiplyScalar(20-pimpj.length());
                    pi.mesh.position.add(pip);
                    pj.mesh.position.add(pjp);
                }

I have tried reversing pimpj with pjmpi while changing pi.velocity, but to no effect. note: I am using three.js