Benchmarking spin-state chemistry in starless core models
  Aims. We aim to present simulated chemical abundance profiles for a variety
of important species, with special attention given to spin-state chemistry, in
order to provide reference results against which present and future models can
be compared. Methods. We employ gas-phase and gas-grain models to investigate
chemical abundances in physical conditions corresponding to starless cores. To
this end, we have developed new chemical reaction sets for both gas-phase and
grain-surface chemistry, including the deuterated forms of species with up to
six atoms and the spin-state chemistry of light ions and of the species
involved in the ammonia and water formation networks. The physical model is
kept simple in order to facilitate straightforward benchmarking of other models
against the results of this paper. Results. We find that the ortho/para ratios
of ammonia and water are similar in both gas-phase and gas-grain models, at
late times in particular, implying that the ratios are determined by gas-phase
processes. We derive late-time ortho/para ratios of ~0.5 and ~1.6 for ammonia
and water, respectively. We find that including or excluding deuterium in the
calculations has little effect on the abundances of non-deuterated species and
on the ortho/para ratios of ammonia and water, especially in gas-phase models
where deuteration is naturally hindered owing to the presence of abundant heavy
elements. Although we study a rather narrow temperature range (10-20 K), we
find strong temperature dependence in, e.g., deuteration and nitrogen
chemistry. For example, the depletion timescale of ammonia is significantly
reduced when the temperature is increased from 10 to 20 K; this is because the
increase in temperature translates into increased accretion rates, while the
very high binding energy of ammonia prevents it from being desorbed at 20 K.
