van der Waals interactions in the ground state of Mg(BH4)2 from density functional theory

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 (P6(1)22) and a closely related Mn(BH4)(2)-prototype phase (P3(1)12) 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.