Next-generation nonlocal van der Waals density functional

In 2004, a seminal paper introduced the first practical non-local van der Waals density functional called vdW-DF. However, since then, the functional form of vdW-DF has remained unchanged. Bringing together different insights from almost two decades of development and testing, we present the next-generation non-local correlation functional called vdW-DF3 [J. Chem. Theory Comput. 16, 5893 (2020)], 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 semi-empirical. We benchmark vdW-DF3 against a large set of well-studied test cases, finding good performance in general and a significant improvement in accuracy at larger separations.