Ice surface and mantle evolution for f_D1 and f_D2 during the protostellar collapse phase. The variation in densities for different trajectories in the pre-collapse phase induces large shifts in the ratios in the very beginning of the collapse. Once the tracer particles reach the protostellar core, the D/H ratios in the ice mantle converge to similar values, as the trajectories reach similar densities. The y-axes differ between the left and right panels.