Density-functional study of the Cr-8 antiferromagnetic ring

We present the results of an ab initio characterization of the electronic and magnetic properties of the Cr-8 molecular ring. The original molecule with formula [Cr(8)F(8)Piv(16)](HPiv=pivalic acid, trimethyl acetic acid) has been approximated by replacing the disordered methyl groups by H atoms, obtaining de facto the formula [Cr8F8(O2CH)(16)]. We find in agreement with experiments that the Cr atoms attain a local spin moment of 3/2, and that the preferred arrangement of Cr atoms in the ground state is antiferromagnetic. By the total energy difference we calculate the value of the isotropic Heisenberg exchange parameter and we show that only after the inclusion of on-site correlation effects in the Cr d orbitals the value deduced by specific heat and torque measurements is recovered.