Structural transitions in metal clusters

Abstract

The methods for first-principles calculation of the structure and dynamics of clusters have now progressed to a point where clusters containing ca. 50 non-transition-metal atoms can be studied. As a paradigm, we studied the energetics of structural transformations in 13- and 55-atom Al clusters, which can assume both perfect icosahedral and cuboctahedral structures. Using the Car–Parrinello (quantum molecular dynamics) formalism, we found Al₁₃ has a unique structure, a slightly distorted icosahedron, but Al₅₅ has several inequivalent but energetically nearly degenerate structures. The degeneracy in Al₅₅ is due to the short range of the effective interatomic interactions in a metallic cluster and should lead to floppiness at finite temperatures. A new accurate procedure for calculating ionization potentials (E_i) and electron affinities (E_ea) within the Car–Parrinello formalism was developed and applied to Al clusters. Unfortunately, it appears that at least for some clusters, most notably for Al₅₅, the E_i and E_ea are very similar for different structural models of this cluster. A formulation and the first tests of a new multigrid-based method for real space electronic structure calculations are briefly described. This method should make possible calculations similar to the above for clusters containing transition metal and/or first-row atoms in the fairly near future.