In order to resolve an outstanding discrepancy between experiment and theory regarding the ground-state structure of Mg(BH4)2, we examine the importance of long-range dispersive interactions on the compound's thermodynamic stability. Careful treatment of the correlation effects within a recently developed nonlocal van der Waals density functional (vdW-DF) leads to a good agreement with experiment, favoring the {\alpha}-Mg(BH4)2 phase (P6122) and a closely related Mn(BH4)2-prototype phase (P3112) over a large set of polymorphs at low temperatures. Our study demonstrates the need to go beyond (semi)local density functional approximations for a reliable description of crystalline high-valent metal borohydrides.
@article{arxiv.1104.1784,
title = {Van der Waals interactions in the ground state of Mg(BH4)2 from density functional theory},
author = {A. Bil and B. Kolb and R. Atkinson and D. G. Pettifor and T. Thonhauser and A. N. Kolmogorov},
journal= {arXiv preprint arXiv:1104.1784},
year = {2011}
}