van der Waals theory and plasmons now and then

An attempt is made to connect my first [1] and last [2] publications. Both happen to concern dispersion or van der Waals (vdW) forces [3]. Spanning over the years, a bridge can be distinguished, where plasmon, physical consistency, and charge conservation are key concepts. Insights gained by the general-purpose method developed by the Rutgers–Chalmers collaboration, the vdW-DF, show that the mentioned key concepts are fruitful, allowing successive improvements in a controlled manner. The first vdW-DF functional is aimed at describing just the pure vdW bond. When applied more broadly, it does not perform quantitatively accurately. For instance, too long bonds are traced bck to deficient account of exchange. The recent functional, vdW-DF-cx [4,5], where ”cx” means that exchange and correlation (xc) are treated consistently, that is, taken to the same order in screening, a plasmon effect, in a perturbative many-body treatment. It does not only give a high accuracy, today used in leading computing codes, but also shows a systematics for further improvements. The talk will illustrate how useful and important the mentioned key concepts are. The vdW-DF method adheres to important conservation rules, materialized by the xc hole.<\p>On the computational side, the vdW-DF-cx functional is implemented in several main-stream DFT packages. Results of extensive tests of recently developed exchange functionals [6], paired with vdW-DF1 and vdW-DF2 correlation functionals, respectively, are summed up into two recommendations: "any of the three functionals optB86b, vdW-DF-cx, or revB86b-DF2 could be expected to perform well in layered systems, primarily based on the excellent agreement with the experimental geometries".<\p>[1] S Lundqvist and A Sjölander: On polarization waves in Van der Waal's crystals, Ark. Fys. 426, 17 (1963). [2] B Lundqvist and A Johansson: Collective aspects on binding energy in van der Waals crystals, Ark. Fys. 26, 268 ( 1964)).<\p>[3] K Berland, V R Cooper, Kyuho Lee, E Schröder, T Thonhauser, P Hyldgaard and B I Lundqvist: Review Article: van der Waals forces in density functional theory: a review of the vdW-DF method, Rep. Prog. Phys. 00 (2015) 000000 (41pp).<\p>[4] K Berland and P Hyldgaard,: Exchange functional that tests the robustness of the plasmon description of the van det Waals density functional, Phys. Rev. B 89 (2014) 035412.<\p>[5] K Berland, C A Arter, V R Cooper, Kyuho Lee, B I Lundqvist, E Schröder, T Thonhauser, and P Hyldgaard: van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions, J. Chem. Phys. 140, 18A539 (2014). <\p> [6] Björkman, T, Testing several recent van der Waals density functionals for layered structures, J Chem. Phys. 141, 074708 (2014).<\p>