Structure and binding in crystals of cage-like molecules: hexamine and platonic hydrocarbons

TL;DR

This paper demonstrates that van der Waals density functional calculations accurately predict the structure and cohesion of cage-like molecular crystals, revealing differences in binding character compared to asymptotic pair potentials.

Contribution

It applies vdW-DF to determine crystal structures of hexamine and platonic hydrocarbons, highlighting differences from asymptotic models and questioning transferability of DFT-D methods.

Findings

vdW-DF accurately predicts lattice parameters and cohesion energies

Full vdW-DF shows enhanced binding at intermediate separations

Asymptotic pair potentials underestimate binding at relevant distances

Abstract

In this paper, we show that first-principle calculations using a van der Waals density functional (vdW-DF), [Phys. Rev. Lett. $\mathbf{92}$, 246401 (2004)] permits determination of molecular crystal structure. We study the crystal structures of hexamine and the platonic hydrocarbons (cubane and dodecahedrane). The calculated lattice parameters and cohesion energy agree well with experiments. Further, we examine the asymptotic accounts of the van der Waals forces by comparing full vdW-DF with asymptotic atom-based pair potentials extracted from vdW-DF. The character of the binding differ in the two cases, with vdW-DF giving a significant enhancement at intermediate and relevant binding separations. We analyze consequences of this result for methods such as DFT-D, and question DFT-D's transferability over the full range of separations.