Density functional theory is used to explore the structure of Au-25(RS)(18). The preferred structure consists of an icosahedral Au-13 core protected by 6 RS-Au-RS-Au-RS units. The enhanced stability of the structure as an anion is found to originate from closure of an eight-electron shell for delocalized Au(6s) electrons. The evaluated XRD pattern and optical spectra are in good agreement with experimental data.