Most of the "first principles" simulations we do are with a theory known as density-functional theory (DFT). In principle it is exact but in practice it relies on an approximation for how electrons interact with each other. We are tackling the issue of the accuracy of DFT through extensive series of studies of small gas phase complexes, molecular crystals, and molecules at solid interfaces. These benchmark studies with techniques such as quantum Monte Carlo and coupled cluster come with extreme computational burdens. However, these benchmarks are essential to establish the accuracy of more traditional methods such as DFT, and help to ensure that the numbers we produce stand the test of time and experiment.
A major challenge for DFT is the accurate description of van der Waals interactions, and London dispersion in particular. London dispersion interactions are ubiquitous in nature contributing to the binding of biomolecules such as DNA, molecular crystals, and adsorption of molecules on surfaces. Many schemes have been developed that allow dispersion to be accounted for within DFT in a more or less approximate manner. One of the most promising and rigorous method is the nonlocal van der Waals density functional (vdW-DF) proposed by Langreth and Lundqvist and co-workers (M. Dion et al., Phys. Rev. Lett. 92, 246401, 2004). We have been working on developing improved versions on the vdw-DF approach, and in particular have developed optB88-vdW, optPBE-vdW, and optB86b-vdW functionals.