Hi SD,nThe UDF with Reflect_Particle(tp,f_normal,ND_ND,f,t, NULL, NULL_FACE) crashes in both serial and parallel run.nThe reflect boundary condition using DEFINE_DPM_BC was first verified for just the DPM simulation . It worked without any issue.nHowever, when reflect UDF code was used in DEFINE_FILM_REGIME with FILM_USER_0 to simulate rebound, the droplets stopped after hitting the wall.nThe Eulerian Wall Film fate for single droplet parcel injected was nEulerian wall film particles: absorbed = 1, splashed = 0, rebound = 1nThough the fate was supposed to be only rebound , there is an absorbed droplet. I suspect the rebound = 1 is only showing because the n_rebound ++ . I guess the droplet is neither rebounding nor being absorbed, since the droplet trajectory stops and there is no film mass.nPlease let me know if there is a way to have only the rebound.nThanks,nJo