Theory of van der Waals bonding: from bulk materials to biomolecules

Doctoral thesis, 2012

Sparse matter is abundant in Nature. It encompasses systems characterized by an intrinsic low density of electrons in sizeable regions, where the van der Waals forces contribute considerably to cohesion. Given the length scale of the problem, a prediction of these materials requires appropriate tools within a quantum-mechanical framework. Density Functional Theory (DFT) has proven to provide a powerful approach to a non-empirical characterization of condensed-matter properties. However, in spite of the successes achieved by its local (LDA) and semilocal (GGA) approximations, the description of the van der Waals bonding was until recently far from being satisfactory. This is related to the incapability of the local and semilocal exchange-correlation energy functionals to capture the effect of charge fluctuations that arise spontaneously in matter and that are coupled by the electrodynamic field. The problem has been successfully addressed by a density functional (vdW-DF) that accounts for the dispersive interactions by introducing a nonlocality in the correlation term. The work presented in this Thesis contributes to shed light on the van der Waals bonding in soft matter by means of the vdW-DF functional. In particular, it investigates three structurally and electronically different systems (namely belonging to the class of bulks, surfaces and biomolecules) in order to test structural characteristics, cohesive energies, physisorption-related properties and corrugation. A major issue when treating large sparse matter systems coincides with the limit introduced by the selfconsistent calculation of the kinetic term which increases the computing time and memory needed. An attempt to speed-up the computations is presented by taking advantage of the Harris scheme, a nonselfconsistent DFT formulation valid for weakly interacting systems. The account of dispersion forces has a direct impact on the structure resulting in an undoubtedly better description of the atom configuration and the morphology of sparse matter. In addition, it is documented that their fingerprint can also be detected in subtle changes of the band structure and the density of states.