Using the study of the low-spin complex [Fe(bpy)₃]²⁺ in the gas phase and in condensed phases as a guideline, we examine different aspects of the application of DFT to the study of transition metal complexes in the framework of spin crossover or related phenomena.

In the iron(II) low-spin complex [Fe(bpy)₃]²⁺, the zero-point energy difference between the ⁵T_2g(t⁴_2ge²_g) high-spin and the ¹A_1g(t⁶_2g) low-spin states, ΔE⁰_HL, is estimated to lie in the range of 2500-5000 cm^-1. This estimate is based on the low-temperature dynamics of the high-spin→low-spin relaxation following the light-induced population of the high-spin state and on the assumption that the bond-length difference between the two states Δr_HL is equal to the average value of ≈0.2 Ã…, as found experimentally for the spin-crossover system. Calculations based on density functional theory (DFT) validate the structural assumption insofar as the low-spin-state optimised geometries are found to be in very good agreement with the experimental X-ray structure of the complex and the predicted high-spin geometries are all very close to one another for a whole series of common GGA (PB86, PW91, PBE, RPBE) and hybrid (B3LYP, B3LYP*, PBE1PBE) functionals. This confirmation of the structural assumption underlying the estimation of ΔE⁰_HL from experimental relaxation rate constants permits us to use this value to assess the ability of the density functionals for the calculation of the energy difference between the HS and LS states. Since the different functionals give values from -1000 to 12000 cm^-1, the comparison of the calculated values with the experimental estimate thus provides a stringent criterion for the performance of a given functional. Based on this comparison the RPBE and B3LYP* functionals give the best agreement with experiment.