We present the first comprehensive comparison between free energies, based on a phonon dispersion calculation within density functional theory, of theoretically predicted structures and the experimentally proposed α (P6(1)) and β (Fddd) phases of the promising hydrogen storage material Mg(BH(4))(2). The recently proposed low-density [Formula: see text] ground state is found to be thermodynamically unstable, with soft acoustic phonon modes at the Brillouin zone boundary. We show that such acoustic instabilities can be detected by a macroscopic distortion of the unit cell. Following the atomic displacements of the unstable modes, we have obtained a new F 222 structure, which has a lower energy than all previously experimentally and theoretically proposed phases of Mg(BH(4))(2) and is free of imaginary eigenmodes. A new meta-stable high-density I4(1)/amd structure is also derived from the [Formula: see text] phase. Temperatures for the decomposition are found to be in the range of 400-470 K and largely independent of the structural complexity, as long as the primary cation coordination polyhedra are properly represented. This opens a possibility of using simple model structures for screening and prediction of finite temperature stability and decomposition temperatures of novel borohydride systems.