Isomers of Al-13 clusters and their interaction with alkali atoms

Global optimization of the geometry of Al-13 cluster using the density-func tional theory and generalized gradient approximation yields two nearly dege nerate isomers having Jahn-Teller distorted icosahedral and decahedral stru ctures. As these two isomers of Al-13 interact with alkali-atoms X (X=Li, N a, K, Rb, and Cs), the Jahn-Teller distortions in the bare cluster isomers disappear in all cases except in Cs. The binding energy of alkali atoms, X to Al-13 systematically decreases from Li to Cs for both the isomers. This is shown to result from a competition between the size and the ionization p otential of the alkali atoms. In addition, the difference in the total ener gies between icosahedral and decahedral structures containing the alkali at oms becomes smaller than that between the bare Al-13 isomers. The vertical ionization potentials of AlxX is larger than that of Al-13 in the decahedra l structure while the opposite is the case with the icosahedral isomer. The above results based on a frozen core approximation were repeated using an all-electron basis. While there are quantitative differences between these results, these are negligible at the present level of theory.