Including van der Waals interactions in first-principles electronic structure calculations

Abstract

Dispersion (van der Waals or vdW) interactions are long-range, non-local in nature, and can be important for understanding and predicting structure and energetics in many systems. Examples of such systems include weakly bound dimers, molecules on surfaces and molecular crystals. Because of the inherent non-locality of these interactions, they are not accounted for by traditional local and semi-local exchange and correlation functionals in density functional theory (DFT). In this thesis, two different approaches to including dispersion interactions in DFT were investigated and implemented. The first approach is based on a recently developed method [1] that maps the DFT ground-state electron density onto a set of maximally-localized Wannier functions (MLWFs). These MLWFs act as fragments of electron density that are used in a pairwise summation of the vdW contribution to the total energy. This contribution is added to the total DFT ground-state energy in a post-processing fashion. The method, as originally proposed, has a number of shortcomings that hamper its predictive power. To overcome these problems, we developed and implemented a number of improvements to it and demonstrated that these modifications give rise to calculated binding energies and equilibrium geometries that are in closer agreement to results of quantum-chemical coupled-cluster calculations. The second approach, known as the vdW density functional (vdW-DF) method, incorporates a non-local vdW term directly into the exchange and correlation functional. Following a recent efficient implementation [2] we coded this approach and a number of vdW functionals (vdW-DF, vdW-DF2, optB88, optPBE) in the ONETEP linear-scaling DFT package, enabling treatment of very large systems that were previously too computationally demanding for such methods. We applied the vdW-DF method to a system of interest for applications in photovoltaics, namely fullerene (C60) molecular crystals, and investigated the effect of including vdW interactions on the relative stability of different crystal structures.