The reaction of Ga-2 with H-2 to form the most stable singlet Ga(mu-H)(2)Ga isomer I observed to occur spontaneously in solid noble gas matrices by Downs has been investigated theoretically with single-and multi-reference ab initio and DFT methods. Early on the reaction coordinate, a spin crossing is expected to move the process from the reactant ground-state triplet surface onto the singlet surface of the product. The singlet direct C-2v Path involves the most favourable TSPH-H activation transition state lying, at CCSD(T), 2.7 kcal/mol above Ga-2 ((1)Sigma(g)(+)) + H-2. (C) 2004 Elsevier B.V. All rights reserved.