Chemistry in Disks. IV. Benchmarking gas-grain chemical models with
  surface reactions
  Abridged: We detail and benchmark two sophisticated chemical models developed
by the Heidelberg and Bordeaux astrochemistry groups. The main goal of this
study is to elaborate on a few well-described tests for state-of-the-art
astrochemical codes covering a range of physical conditions and chemical
processes, in particular those aimed at constraining current and future
interferometric observations of protoplanetary disks. We consider three
physical models: a cold molecular cloud core, a hot core, and an outer region
of a T Tauri disk. Our chemical network (for both models) is based on the
original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and
a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The
benchmarking is performed with the increasing complexity of the considered
processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with
accretion and desorption, and (3) the full gas-grain model with surface
reactions. Using atomic initial abundances with heavily depleted metals and
hydrogen in its molecular form, the chemical evolution is modeled within 10^9
years. The time-dependent abundances calculated with the two chemical models
are essentially the same for all considered physical cases and for all species,
including the most complex polyatomic ions and organic molecules. This result
however required a lot of efforts to make all necessary details consistent
through the model runs, e.g. definition of the gas particle density, density of
grain surface sites, the strength and shape of the UV radiation field, etc. The
reference models and the benchmark setup, along with the two chemical codes and
resulting time-dependent abundances are made publicly available in the
Internet: http://www.mpia.de/homes/semenov/Chemistry_benchmark/home.html
