Based on the local spin theory of Davidson and Clark, the one-body reduced density matrix ρ was written as S 1 / 2 PS 1 / 2 and then a simple program was written by us for calculating the local spin for molecules. After check the validity of the program by sample of MnCl 2 (H 2 O) 2 , the local spin and the coupling magnetic constant J were calculated for the dimer [Fe 2 Cl 6 O] 2 - with different conformations in crystals [BzPh 3 P] 2 [Fe 2 Cl 6 O] (a), [Fe(bpy) 3 ][Fe 2 Cl 6 O] (b), [Hpy] 2 [Fe 2 Cl 6 O].py (c), [Ph 4 As] 2 [Fe 2 Cl 6 O] (d), and [Ph 4 P] 2 [Fe 2 Cl 6 O] (e), and the variation of magnetic properties with its geometry were discussed by the structure model based on the [Fe 2 Cl 6 O] 2 - dimer in crystal [Hpy] 2 [Fe 2 Cl 6 O].py (c).