Towards superior van der Waals density functionals for molecular crystals

Abstract

Ubiquitous van der Waals (vdW) interactions play a subtle yet crucial role in determining the precise atomic arrangements in solids, particularly in molecular crystals where these weak forces are the primary link between constituent building blocks. Within density functional (DF) theory, the most natural approach for addressing vdW forces is the use of vdW-inclusive density functionals. Through a detailed analysis of the underlying formalism, we have developed a computational scheme that combines vdW functionals of type DF1 and DF2 and serves as a well optimizable tool to improve the theoretical description and prediction of molecular crystals and other sparse materials. The proof of principle is demonstrated by our consideration of the molecular crystals from the X23 dataset.

Figures (3)

• Figure 1: The average relative error for the unit cell volume of the X23 crystals for the considered vdW-DF-optB88-$Z_{ab}$, vdW-DF-B86R-$Z_{ab}$, and vdW-DF-cx-$Z_{ab}$ functionals. As the reference data, the cell volumes $V_{\rm el}^{\rm ref}$ from Ref. Dolgonos2019 are taken.
• Figure 2: The two-dimensional map of the average errors for unit cell volumes and lattice energies of the X23 molecular crystals. The results obtained for original points (labeled with red dots) are interpolated to $0.7114 < \kappa < 1.0031$ and $-2.25 < Z_{ab} < -1$.
• Figure 3: The relative error for cell volumes (left) and lattice energies (right) of the X23 molecular crystals calculated by PBE-MBD Loboda2018, PBE0-MBD Loboda2018, PBE-D3 (with the Becke-Johnson damping function) Loboda2018, rev-vdW-DF2 (identical to vdW-DF-B86R-1.8867 in our notations), vdW-DF-B86R-1.8106, and vdW-DF-B86R-1.8791 functionals. As the reference data, the cell volumes $V_{\rm el}^{\rm ref}$ and the lattice energies $E_{\rm latt}^{\rm ref, exp}$ from Ref. Dolgonos2019 are employed, as fairly corresponding to pure electronic results.