Equilibrium geometries, electronic structure and magnetic properties of small manganese clusters

The equilibrium geometries, electronic structure and magnetic properties of small Mn clusters consisting of up to five atoms have been calculated self -consistently using first principles molecular orbital theory. The electron -electron interaction has been accounted for using the local spin density a nd generalized gradient approximation to the density functional theory. The atomic orbitals forming the molecular orbital have been represented separa tely by Gaussian and numerical basis sets. Two different computer codes (Ga ussian 94 and DMOL) were used to check the numerical consistency of our cal culations. Mn-2 is found to be a weakly bound van der Waals molecule and it s binding energy depends sensitively on the choice of basis set as well as the form of the exchange-correlation potential. The binding energies are le ss sensitive to these approximations in larger clusters. The binding improv es with cluster size, but remains significantly lower than those in other t ransition metal clusters. The equilibrium geometries are fairly compact and symmetric although other isomers with distorted geometries and with nearly the same energy as that of the ground state do exist for Mns. The clusters also exhibit a variety of low-lying spin multiplicities, bur the ground st ate spin configuration is ferromagnetic with a magnetic moment of 5 mu(B)/a tom. This not only contrasts with its bulk behaviour which is antiferromagn etic, but also differs from the behaviour in other transition-metal cluster s where the magnetic moments/atom are always less than the free-atom value. The results are compared with available experiments on matrix isolated Mn clusters.