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Dielectronic recombination (DR) of singly charged ions is a reaction pathwaythat is commonly neglected in chemical models of molecular clouds. In thisstudy we include state-of-the-art DR data for He$^+$, C$^+$, N$^+$, O$^+$,Na$^+$, and Mg$^+$ in chemical models used to simulate dense molecular clouds,protostars, and diffuse molecular clouds. We also update the radiativerecombination (RR) rate coefficients for H$^+$, He$^+$, C$^+$, N$^+$, O$^+$,Na$^+$, and Mg$^+$ to the current state-of-the-art values. The new RR data haslittle effect on the models. However, the inclusion of DR results insignificant differences in gas-grain models of dense, cold molecular clouds forthe evolution of a number of surface and gas-phase species. We find differencesof a factor of 2 in the abundance for 74 of the 655 species at times of$10^4$--$10^6$ years in this model when we include DR. Of these 74 species, 16have at least a factor of 10 difference in abundance. We find the largestdifferences for species formed on the surface of dust grains. These differencesare due primarily to the addition of C$^+$ DR, which increases the neutral Cabundance, thereby enhancing the accretion of C onto dust. These results may beimportant for the warm-up phase of molecular clouds when surface species aredesorbed into the gas phase. We also note that no reliable state-of-the-art RRor DR data exist for Si$^+$, P$^+$, S$^+$, Cl$^+$, and Fe$^+$. Moderncalculations for these ions are needed to better constrain molecular cloudmodels.

Molecular cloud chemistry and the importance of dielectronic recombination