  We present libvdwxc, a general library for evaluating the energy and
potential for the family of vdW-DF exchange--correlation functionals. libvdwxc
provides an efficient implementation of the vdW-DF method and can be interfaced
with various general-purpose DFT codes. Currently, the GPAW and Octopus codes
implement interfaces to libvdwxc. The present implementation emphasizes
scalability and parallel performance, and thereby enables \textit{ab initio}
calculations of nanometer-scale complexes. The numerical accuracy is
benchmarked on the S22 test set whereas parallel performance is benchmarked on
ligand-protected gold nanoparticles
($\text{Au}_{144}(\text{SC}_{11}\text{NH}_{25})_{60}$) up to 9696 atoms.
