libvdwxc is a general library for evaluating energy and potential 
for exchange-correlation (XC) functionals from the vdW-DF family 
that can be used with various of density functional theory (DFT) 
codes. This work was inspired by success of libXC, a library for 
local and semilocal XC functionals. At the moment, libvdwxc provides 
access to the DF1, DF2, and CX functionals and interfaces for GPAW 
and Octopus. The library has been tested with respect to the S22 
test set, various bulk properties of metals and semiconductors, 
and surface energies.