emphasis on any changes made to them since they were originally published. Section 4 describes the expansion of data that is now provided by the ExoMol database. Section 5 gives a formal description of the new data structures implemented to cover this provision and to provide the API functionality. Section 6 briefly describes the utility programs available as part of the ExoMol database and Section 7 the ExoMol website. Finally we discuss future prospects for the database.

Database coverage
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The ExoMol project aims at complete coverage of the spectroscopic properties of molecules which are deemed to be important in hot astrophysical environments. Coverage concerns (a) the molecular species considered, including isotopologues; (b) the frequency range considered and (c) the upper temperature range for which the data is reasonably complete. Both the required temperature and frequency range completeness are to some extent a judgement on what is required for astronomical and other studies. For example, a molecule like nitric acid (HNO$_3$) will dissociate at a relatively low temperature so coverage above about 500 K is unlikely to be important. On the other hand, several diatomics such SiO and CO are known to feature in stellar spectra, and coverage to temperatures 