  To prepare for the unprecedented spatial and spectral resolution provided by
ALMA and Herschel/HIFI, chemical models are being benchmarked against each
other. It is obvious that chemical models also need well-constrained
observations that can serve as references. Photo-dissociation regions (PDRs)
are particularly well suited to serve as references because they make the link
between diffuse and molecular clouds, thus enabling astronomers to probe a
large variety of physical and chemical processes. At a distance of 400 pc (1"
corresponding to 0.002 pc), the Horsehead PDR is very close to the prototypical
kind of source (i.e. 1D, edge-on) needed to serve as a reference to models.
