Ab initio study of the structure and polarizability of sulfur clusters, Sn (n=2-12)

The structure and the dipole polarizabilities of S-n clusters (n = 2-12) have been calculated using density functional theory within the B3LYP approximation and conventional ab initio Hartree-Fock (HF) and coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) methods. The results show that the binding energy per atom increases with the size of the cluster and reaches the asymptotic limit for a relatively small n value. There is an excellent agreement between B3LYP and CCSD(T) data in predicting the energy of the disproportionation reaction 2S(n) --> Sn-1 + Sn+1, which indicates that S-2, S-6, and S-8 are especially stable, in agreement with the experiment. (alpha) increases with n and linearly correlates with the molecular volume. (alpha)/n increases with n and reaches the asymptotic limit per n --> infinity from below, contrary to what happens in small semiconductor and metallic clusters. A well-defined correlation between (alpha) and hardness is not found, while the (alpha (n)) - n(alpha (1)) difference value linearly correlates with the atomization energy. In the sulfur clusters, the minimum polarizability principle does not hold, the lone-pair electron polarizability being more diffuse, hence more polarizable, in the cluster than in the free atom. Pure vibrational effects on (alpha) are negligible.