Density functional theory with plane wave basis set and genetic algorithm methods were used to investigate the structural and electronic properties of neutral copper clusters Cu_n with n up to 20. Properties involving the cluster size effect of average bond lengths, average coordination numbers, binding energies, second differences in total energy, ionization potentials and electron affinities were presented in detail. Changes from two dimensional to three dimensional structures were found between Cu_3～6 and Cu_7～20. Magic number clusters at n＝8, 13 and 20 obtained from these results are in good agreement with experimental observation. The trends for the average bond lengths, average coordination numbers and binding energies almost monotonously increase with the size increment to the limits in bulk. The clear odd-even alternation for ionization potentials, electron affinities and second differences in total energy indicated the stability of electronic close shell existed in the range studied.

Key words: genetic algorithm, Gupta-type potential, density functional, Cu cluster