-5  0  5  10  15  20  0  0.5  1  1.5  2  2.5 ionic z-coord. (a 0 ) time (ps) Na$_{6}$ leaves box E $^{0}$$_{kin}$=8.16 eV Na O Mg -5  0  5  10  15  20 ionic z-coord. (a 0 ) E $^{0}$$_{kin}$=0.816 eV Na$_{6}$ leaves box -5  0  5  10  15  20 ionic z-coord. (a 0 ) E $^{0}$$_{kin}$=0.0816 eV -10 -5  0  5  10 -10 -5  0  5  10 ionic y-coord. (a 0 ) ionic x-coord. (a$_{0}$) -10 -5  0  5  10 ionic y-coord. (a 0 ) -10 -5  0  5  10 ionic y-coord. (a 0 ) FIG. 6: Left : Time evolution of the ionic z-coordinates of Na$_{6}$ approaching MgO(001). Right : Projection of the Na and MgO trajectories into the x-y-plane; the figures are vertically sorted by increasing initial kinetic energy : Soft deposition (top), robust deposition (center), reflection (top). FIG. 5: Snapshots of the time evolution for three cases of col- lisions of Na$_{8}$ on MgO(001) with different initial kinetic ener- gies E$_{kin}$ . Upper panel : Soft deposition with E 0 kin = 0 . 109 eV. Middle panel : Robust deposition with E 0 kin = 1 . 09 eV. Lower panel : Reflection with E 0 kin = 10 . 9 eV. 7 2. The case of strongly oblate Na$_{6}$ Results for the deposition of Na$_{6}$ are shown in figure 6. The impact energy is varied and all three cases start from the same initial configuration where the top ion of Na$_{6}$ (see section II F) is facing away from the substrate and the fivefold ring is parallel to the surface. The gen- eral features are similar to the case of Na$_{8}$. One observes a large internal excitation of the cluster while compara- tively little perturbation goes to the substrate and there is again the clear distinction between deposition for lower impact energies and reflection for higher ones. But there are several interesting differences in detail. Most of all, there is a strong lateral drift in all cases. Indeed the pentagonal ring of Na$_{6}$ does not match the rectangular As noted above, when deposited, the Na$_{6}$ experiences a sizable drift due to the mismatch of its structure with the crystalline structure of MgO. One thus expects that direction and strength of the lateral motion depend sen- sitively on the initial position and orientation of Na$_{6}$ rel- ative to the surface. Indeed figure 7 shows the trajec- tory of the center-of-mass of the Na$_{6}$ cluster projected onto the x - y -plane for three different initial orientations. There are obvisouly dramatic differences. The cluster is kicked to a strong lateral motion for initial impact at structure of MgO, which hinders it from fully accomodat- ing the attractive oxygen sites. Thus one or two corners of the pentagon are bent up during the deposition pro- cess, and this, in turn, induces a sizeable lateral momen- tum (see right panels of figure 6), and a strong perturba- tion of the pentagon, as can be deduced from the motion of z -coordinates shown in the left panels. In the case of the robust deposition, the cluster even rolls over the surface. The stronger lateral excitation leaves also some- what more perturbation to the substrate than in the case of Na$_{8}$, as may be spotted when comparing figures 6 and 4. That will become more obvious when checking ener- gies in section III D. Finally, it is interesting to note that the cluster orientation is also reverted in the case of re- flection. The z coordinates (upper left panel in figure 6) suggest a process where the ring and the former top ion are reflected ”independently” such that the topping ion is departing ”behind/after” the ring, and thus reverting the cluster orientation.