The structural, electronic, and dynamic properties of the organolithium compd. C2H2Li2 were studied via ab initio mol. dynamics simulations based on the Car-Parrinello method. Dynamic simulated annealing techniques applied to search for the low-energy configurations resulted in a structure that is not a dilithioethylene isomer as suggested by the stoichiometric formula but an acetylenic deriv. that can be visualized geometrically as a complex of lithioacetylene with Li hydride HC2Li.HLi. However, the ground state electronic structure is more suggestive of an ionic complex (H-C:C)-.(Li+H-Li+) in which the linear anion HCC- binds to the two Li cations in the triangular complex Li+H-Li+. Several ethylene-like isomers were identified via high-temp. quenches, but these invariably turned out to lie at high energies (.apprx.>30 kcal/mol). Anal. of the high-temp. dynamics indicated that ethylene-like isomers are always unstable toward an intramol. H migration mediated via a Li hydride unit. The direct observation of these intramol. rearrangement reactions revealed the role of the Li atoms as H transfer reagents and confirmed the importance of Li hydride as an intermediate species. [on SciFinder (R)]