Modelling Aluminium Clusters with an Empirical Many-Body Potential

An empirical two-plus-three-body atomistic potential, derived by fitting experimental data pertaining to bulk aluminium, has been applied to study the structures and growth patterns of small aluminium clusters. The high dimensionality of the nuclear configuration space for clusters results in an extremely large number of isomers – local minima on the potential energy hypersurface. Global optimisation (i.e. searching for the lowest energy structure) was carried out, using Random Search and Monte Carlo Simulated Annealing methods, for Al2 Al20. The results of random searching have been used to put lower bounds on the number of minima for these nuclearities and the efficiency of the Monte Carlo Simulated Annealing approach has been demonstrated. Detailed results using both search methods are presented for Al19. Comparisons are made with the results of previous calculations – using electronic structure and empirical potential methods and good agreement is generally observed. While many of the global minima correspond to structures (mostly based on icosahedral growth) which are also global minima for Lennard-Jones or Morse clusters, a number of new structures have been identified for AlN clusters – notably for N = 9, 16, 17, 18 and 20.