Crystals of cage-like molecules: A van der Waals density functional study

A recently developed ab initio van der Waals density functional (vdWDF) [1] includes long-range dispersion forces at an electronic level. It promises to extend large-scale electronic structure calculations to the realm of soft and sparse matter. Consequently, it has spurred many studies on supramolecular systems including molecular dimers, physisorption systems, and polymer bundles [2]. We investigate the performance of vdW-DF for molecular crystals of hexamine and the platonic hydrocarbons cubane (Fig. 1) and dodecahedrane [3]. These molecules are highly symmetric and allow for straightforward comparison between different theoretical approaches and experiments. The calculated lattice parameters and cohesion energy of these crystals (Fig. 2 for cubane) agree well with experiments. A weak overestimation of binding separations are attributed to the choice of exchange used in combination with the non-local correlation of vdW-DF.