  The dominant majority of the hundreds of available spin-crossover compounds,
including the technologically most promising ones, are based on the
Earth-abundant metal iron, making these switches particularly appealing in
terms of sustainable technology. Furthermore, it has recently been established
that these materials may be synthesized using the techniques of Green
Chemistry. Spin crossover in transition metal complexes can be induced by a
change of temperature, by the application of an external pressure, or a
magnetic field, and also by photoexcitation. Given the wide variety of
functionalities to which these bistable photomagnetic systems could give
access, they may be viewed as the prototypes of molecular-scale optomagnetic
switches. Hence, in response to the growing demand for storing and treating
increasingly-dense information, much effort has been devoted over several
decades to the design of transition metal materials exhibiting spin crossover
in technologically-accessible temperature ranges. The cornerstone for designing
new efficient photoactive spin-crossover materials remains the ability to
predict the magnetic behaviour and the photoresponse of any first-row
transition complex, and the manner in which its properties are influenced by
its environment. This is a challenge for the inorganic chemist and also, most
notably, for the theoretical and the computational chemist. This chapter gives
an overview of the issues tied to the application of DFT and TD-DFT to the
characterisation of transition metal complexes in the framework of
spin-crossover and related phenomena.
