Lowest-energy structures of (C60)nX (X=Li+,Na+,K+,Cl-) and (C60)nYCl (Y=Li,Na,K) clusters for n≤13

Basin-hopping global optimization is used to find likely candidates for the lowest minima on the potential energy surface of (C-60)(n)X (X=Li+,Na+,K+,Cl-) and (C-60)(n)YCl (Y=Li,Na,K) clusters with nless than or equal to13. The energy is evaluated using the Girifalco form for the C-60 intermolecular potential along with a polarization potential, which depends on the first few nonvanishing C-60 multipole polarizabilities. We find that the ions occupy interstitial sites of a (C-60)(n) cluster, the coordination shell being triangular for Li+, tetrahedral for Na+ and K+, and octahedral for Cl-. When the required coordination site does not exist in the corresponding (C-60)(n) global minimum, the lowest minimum of the doped system may be based on an alternative geometry. This situation is particularly common in the Cl- complexes, where the (C-60)(n) global minima with icosahedral packing change into decahedral or closed-packed forms for the ions. In all the ions we find a significant binding energy for the doped cluster. In the alkali chloride complexes the preferred coordination for the diatomic moiety is octahedral and is basically determined by the Cl- ion. However, the smaller polarization energies in this case mean that a change in structure from the (C-60)(n) global minimum does not necessarily occur if there is no octahedral site. (C) 2004 American Institute of Physics.