Towards a working density-functional theory for polymers:: First-principles determination of the polyethylene crystal structure

Equilibrium polyethylene crystal structure, cohesive energy, and elastic constants are calculated by density-functional theory applied with a recently proposed van der Waals (vdW) density functional (DF) for general geometries [Phys. Rev. Lett. 92, 246401 (2004)] and with a pseudopotential-plane-wave scheme. The vdW-DF with its account for the long-ranged van der Waals interactions gives not only a stabilized crystal structure but also values of the calculated lattice parameters and elastic constants in quite good agreement with experimental data, giving promise for successful application to a wider range of polymers.