The precise atomic structure of the recently synthesized "magic cluster" Au-20(SR)(16) is predicted using ab initio calculations and global-minimum searches. The cluster contains a prolate Au-8 core and four level-3 extended staple motifs (-RS-Au-RS-Au-RS-Au-RS-). The simulated optical absorption spectra of the lowest-energy structures are in good agreement with the measured spectrum. The Au-20(SR)(16) cluster, with a Low Au/SR ratio of 1.25:1, may represent a structural evolution from core-free homoleptic clusters [Au(SR)](N) to core-stacked clusters.