Energy level structure, autoionization rates, cross-sections for excitation, ionization, charge exchange, and recombination all enter into model calculations. In geophysical studies of the atmosphere, for example, the emission features of atomic oxygen associated with branching ratios for decay from state are related to the abundance of oxygen in the thermosphere. Astrophysics has a longstanding need for large amounts of data.
Atomic structure is an area where accuracy is extremely important.
In spectroscopy, observations are related to energy differences. It is these differences that need to be predicted to spectroscopic accuracy, which, in many applications, may be defined as a fraction of a cm -1 . This translates to an error of less than 4.5 au. Even for relatively small systems, this requirement may present a considerable challenge. A number of physical effects may need to be included to achieve the desired accuracy.