Based on the local spin theory of Davidson and Clark, the one-body reduced density matrix p was written as (SPS1/2)-P-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 MnCl2(H2O)(2), the local spin and the coupling magnetic constant J were calculated for the dimer [Fe2Cl6O](2-) with different conformations in crystals [BzPh(3)P](2)[Fe2Cl6O] (a), [Fe(bpy)(3)][Fe2Cl6O] (b), [Hpy](2)[Fe2Cl6O] (.) py (c), [Ph4As](2)[Fe2Cl6O] (d), and [Ph4P](2)[Fe2Cl6O] (e), and the variation of magnetic properties with its geometry were discussed by the structure model based on the [Fe2Cl6O](2-) dimer in crystal [Hpy](2)[Fe2Cl6O] (.) py (c) (C) 2004 Elsevier B.V. All rights reserved.