  The van der Waals dispersion coefficients of a set of polycyclic aromatic
hydrocarbons, ranging in size from the single-cycle benzene to circumovalene
(C66H20), are calculated with a real-time propagation approach to
time-dependent density functional theory (TDDFT). In the non-retarded regime,
the Casimir-Polder integral is employed to obtain C6, once the dynamic
polarizabilities have been computed at imaginary frequencies with TDDFT. On the
other hand, the numerical coefficient that characterizes the fully retarded
regime is obtained from the static polarizabilities. This ab initio strategy
has favorable scaling with the size of the system - as demonstrated by the size
of the reported molecules - and can be easily extended to obtain higher order
van der Waals coefficients.
