  We have investigated the lowest-energy structures and electronic properties
of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with
local density approximation. The small Au$_n$ clusters adopt planar structures
up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a
structural transition from tabular cage-like structure to compact
near-spherical structure is found around n=15. The most stable configurations
obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of
icosahedral or fcc-like, while the electronic density of states sensitively
depend on the cluster geometry. Dramatic odd-even alternative behaviors are
obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of
gold clusters. The size evolution of electronic properties is discussed and the
theoretical ionization potentials of Au$_n$ clusters compare well with
experiments.
