Aluminum borohydride is studied by means of ab initio calculations taking into account correlation energy. For the (eta2,eta2,eta2) structure, the rotational barrier associated with the conrotation of the three BH4- groups around the Al-B axis is found to be very low (0.2 kcal.mol-1). A rationalization based on an orbital analysis is given. Changing the coordination mode (eta2 --> eta3) of one borohydride group leads to the (eta2,eta2,eta3) structure which is the transition state of the hydrogen exchange mechanism. This is a low energy process (2.2 kcal.mol-1) which explains that the coalescence could not be experimentally observed. Another change in the coordination mode (eta2 --> eta1) of one borohydride group leads to the (eta1,eta2,eta2) structure which is the transition state of the dissociation mechanism Al(BH4)3 --> AlH(BH4)2 + BH3. This is a high-energy process (24.8 kcal.mol-1) probably because the products are electronically deficient.