1 Introduction

The UMIST Database is widely used by the scientific community in modelling chemical kinetics in the interstellar medium. In this paper, we report on the latest release of the database and on electronic access to our full range of data and related codes. In recent years, some very large models, often containing up to 10 000 reactions, have been published. Our latest release takes a much more circumspect approach with the number of reactions extended by 249 compared to the previous release (Millar et al. 1997), to a total of 4113. Large systems often take a generic approach to the rate coefficients and chemical reactions included by extending particular reactions applicable to smaller species to much larger systems. While this is a useful approach in determining whether or not large molecules may form, it is not based generally on thermodynamics which can affect particular reactions.

In recent years, there has been an increasing need to model the chemical kinetics of hot circumstellar envelopes and interstellar shocks, with the result that we have reviewed the temperature dependence and temperature range validity of each reaction. Although not yet totally accurate, we believe this new piece of data should make models noticeably more reliable. In addition, we have included a number of three-body reactions which are of particular importance in high density environments, such as protoplanetary disks. In Sect. 2, the new conventions adopted to tabulate the species and their possible isomers are explained. Also, a few words of caution are given on the data, mainly concerning the lack of distinction between certain isomers. Section 3 describes the content of the database and the new format used. Section 4 gives details on the new data that have been included in our online database set. Finally, Sect. 5 reports on the use of the newly developed Java applet which has been designed to enable the user to select and save to a local disk particular data.