DFT calculations of the mononuclear Fe(II) spin crossover complexes [Fe(L)](2+) (L = ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine})), ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) abbreviated as (66), (56) and (55) have been performed in order to explain the observed spin transition temperature differences. The complexes differ in the size of two chelate rings, revealing two six-membered, one six-membered and one five-membered, and two five membered rings for (66), (56) and (55), respectively. Calculations of the electronic energy differences ΔEel = Eel(HS) - Eel(LS) with the use of the basis set TZVP with B3LYP*, PBE, TPSS and TPSSh functionals reproduced the experimentally observed trends. The best reproduction of bond distances is obtained using the TPSSh functional. The Continuous Shape Measure (CShM) analysis of the optimised structures of all six spin isomers revealed the most significant distortion from the trigonal prism for the low-spin (66) system, which has the lowest spin transition temperature. The corresponding trigonal twist is proposed to be the main cause of releasing strain that is induced by the size of two fused chelate rings. Different conformers of low-spin and high-spin (66) systems were modelled using the TPSSh/TZVP method, including the calculations of transition states of conformational rearrangements in both spin isomers. A normal co-ordinate analysis was performed for all six spin isomers. This allows the assignment of previously reported Raman marker bands to specific modes of the (66) system. The estimate of the vibrational contribution to the spin transition entropy revealed values of 50-60 J K(-1) mol at room temperature for all three complexes.