Next-Generation Nonlocal van der Waals Density Functional

The
fundamental ideas for a nonlocal density functional theorycapable
of reliably capturing van der Waals interactionswere already
conceived in the 1990s. In 2004, a seminal paper introduced the first
practical nonlocal exchange–correlation functional called vdW–DF, which
has become widely successful and laid the foundation for much further
research. However, since then, the functional form of vdW–DF
has remained unchanged. Several successful modifications paired the
original functional with different (local) exchange functionals to
improve performance, and the successor vdW–DF2 also updated
one internal parameter. Bringing together different insights from
almost 2 decades of development and testing, we present the next-generation
nonlocal correlation functional called vdW–DF3, in which we
change the functional form while staying true to the original design
philosophy. Although many popular functionals show good performance
around the binding separation of van der Waals complexes, they often
result in significant errors at larger separations. With vdW–DF3,
we address this problem by taking advantage of a recently uncovered
and largely unconstrained degree of freedom within the vdW–DF
framework that can be constrained through empirical input, making
our functional semiempirical. For two different parameterizations,
we benchmark vdW–DF3 against a large set of well-studied test
cases and compare our results with the most popular functionals, finding
good performance in general for a wide array of systems and a significant
improvement in accuracy at larger separations. Finally, we discuss
the achievable performance within the current vdW–DF framework,
the flexibility in functional design offered by vdW–DF3, as
well as possible future directions for nonlocal van der Waals density
functional theory.