The present PhD work takes place in the framework of the spin-crossover phenomenon. Spin-crossover is a spin-state switching phenomenon between a high-spin and a low-spin electronic configuration in a first-row transitional metal complex. The spin-crossover phenomenon is widely recognized as an example of molecular bistability. The most studied spin-crossover compounds are Fe(II) hexacoordinated complexes that have six N atoms coordinating the Fe(II) ion. Spin-crossover is an entropy-driven phenomenon: it takes the complex of interest from the low-spin electronic ground state, which is exclusively populated at low temperatures, to the high-spin metastable state, which becomes exclusively populated at elevated temperatures.

Theoretical chemistry find many applications in the area of coordination chemistry. In the last few years, it became more and more applied to the study of spin-crossover and related phenomena but it still encounters significant problems in predicting the high-spin - low-spin transition. Most theoretical methods indeed fails at describing the differential electronic correlation associated with the electronic rearrangement within the compact 3d shell which accompanies the transition, and hence at predicting the tiny high-spin - low-spin energy difference in spin-crossover complexes.

A new approach has been tested during this PhD thesis, in order to come with accurate estimates for high-spin - low-spin energy difference in spin-crossover complexes, and the results obtained are in line with the expected magnetic behaviour for all of the studied complexes.