Self-consistent implementation of the van der Waals density functional into the FHI-aims electronic structure code

Abstract

Van der Waals (vdW) interactions play an important role in the bonding of sparse biological matter. However, standard Density Functional Theory (DFT), which is commonly used to simulate solid-state systems, fails to properly describe vdW interactions. This is because the common generalized-gradient or local density approximations of electron exchange and correlation fail to account for nonlocal electron correlations which are the cause of vdW forces. Schemes for restoring van der Waals interactions to the framework of DFT are currently the subject of much attention. This work describes the implementation of the Langreth-Lundqvist Van der Waals Density Functional (vdW-DF) into the electronic structure code FHI-aims. The vdW-DF has been implemented in numerous other codes successfully and has been shown to greatly increase the accuracy of calculations with systems in which vdW interactions are prominent. Implementation of vdW-DF into FHI-aims, however, will require special considerations due to the use of all-electron density rather than pseudopotentials. Among these, a method is developed to allow accurate interpolation of all-electron density from an adaptive octree grid. The implementation is tested in calculations of the interaction energies of noble gas dimers and the S22 set of biological complexes. The accuracy achieved is comparable to that of other implementations and is a significant improvement over standard DFT. This allows for the prospect of simulations of important biological systems with vdW-DF in FHI-aims. The computational performance of the algorithm is assessed and options for continuing development are outlined.