The optimized geometries, vibrational frequencies and thermodynamic functions of the singlet and quintet spin states of three ferrous complexes: Fe(1,10-phenanthroline)(2)(NCS)(2) [Fe(2-picolylamine)(3)](2+) and [Fe(bis(1,4,7-triazacyclononane)(2)](2+) were calculated by the BP86/6-31G* density functional method. The characteristics of the spin-crossover transitions in solution were simulated by the mean-field approximation. The model uses the ideal gas enthalpy and entropy functions based on DFT results and two parameters: A, which scales the electronic energy difference DeltaE(elec) and mimics solute-solvent interactions and F, which describes interactions between solute molecules. Since the fitted quantities are unique functions of the two parameters their experimental values could be exactly recovered. (C) 2003 Elsevier Science B.V. All rights reserved.