optB88vdW

optB88-vdW is a nonlocal van der Waals density functional used within density functional theory to account for dispersion interactions. It combines an optimized exchange component, optB88, with the nonlocal correlation term of the vdW-DF family to provide improved descriptions of weak intermolecular forces, adsorption phenomena, and layered materials.

Developed by Jan Klimeš, David Bowler, and Angelos Michaelides in the early 2010s, optB88-vdW is one of several optimized exchange vdW functionals designed to improve upon the original vdW-DF1 and vdW-DF2 functionals. The optB88 exchange is parameter-tuned to yield more accurate binding energies and geometries when coupled with the vdW nonlocal correlation.

The functional has been widely used for studying molecular adsorption on surfaces, interlayer binding in layered materials (such as graphite and transition-metal dichalcogenides), and molecular crystals. Compared with earlier vdW-DF implementations, optB88-vdW generally provides better agreement with experimental binding energies and lattice parameters for a broad class of systems.

optB88-vdW is implemented in several electronic structure codes, including VASP, Quantum ESPRESSO, GPAW, ABINIT, and CP2K, among others. Users should be aware that, as with all nonlocal functionals, the results can depend on the choice of pseudopotentials or projector augmented–wave datasets, grid density, and convergence settings. While it improves many weak-interaction predictions, it may overbind certain systems or underbind others, so cross-checks with experiment or higher-level methods are recommended.