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When people refer to the initial conditions for star formation, they often
apprehended them as a stage in which the molecular cloud has already
fragmented into more or less distinct starless (prestellar) cores. However,
this stage can only be seen as initial from the point of view of the
terminal prestellar collapse. At the same time, it is by no means initial
from the chemical point of view. Available observations of chemistry in
diffuse interstellar clouds show that their molecular composition is
surprisingly rich, so that the chemical evolution in prestellar cores hardly
starts from purely atomic abundances. This may have sizeable consequences
for the interpretation of future observations with high angular resolution
and/or sensitivity. We developed a chemo-dynamical model for a pre-starless
stage of a prestellar core evolution, with a special emphasis on gas and
dust thermal balance, photoreactions, including H2 photodissociation, and
surface chemistry, including stochastic treatment of H2 formation. We study
how different scenarios of a starless core formation are reflected in its
"initial" chemical composition. Effects of the environment are also
illustrated.
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