AB-INITIO CI DETERMINATION OF THE EXCHANGE COUPLING-CONSTANT OF DOUBLY-BRIDGED NICKEL(II) DIMERS

Ab initio DDCI2 (difference-dedicated configuration interaction) calcu lations are performed on the exchange coupling constant of the doubly- bridged Ni(II) complexes [Ni(en)(2)Cl](2+)(2) and [Ni(terpy)(N-3)](2+) (2), which are modeled by substituting the external ligands with ammon ia groups. The variational CI space is selected on the grounds of the effective Hamiltonian theory and includes all the second-order contrib utions to the difference between the lowest quintet, triplet, and sing let states. Both complexes are found to be ferromagnetic, with couplin g constants of 1.8 and 21.1 cm(-1), in good agreement with the experim ent. A transformation of the molecular orbitals is also proposed for l arge systems, enabling the molecular orbital set to be significantly t runcated-as well as the file of two-electron integrals and the DDCI2 s pace-with no loss of efficiency.