THE REACTIONS AL(P-2)-2-]ALH2(1(2)A',2(2)A')-]ALH2(X(2)A(1)) OR ALH(X-1-SIGMA(+))+H - UNUSUAL CONICAL INTERSECTIONS AND POSSIBLE NONADIABATIC RECROSSING(H)

The energies and derivative couplings are computed in the vicinity of the 1(2)A'-2(2)A' seam of conical intersections for AlH2. It is shown that the reaction path for the decomposition of excited vibrational le vels of AlH2(X(2)A(1)) to Al + H-2 passes quite close to the minimum e nergy crossing point (MECP), the minimum energy point on the seam of c onical intersections. Near the MECP the seam of conical intersections exhibits an unusual trifurcation with a C-2v portion dividing into a b ranch that preserves C-2v symmetry and two symmetry equivalent branche s that have only C-s symmetry.