  The structure of thiolate-protected gold cluster Au38(SR)24 has not been
determined experimentally and the best available signature is its measured
optical spectrum. Using this signature and energetic stability as criteria and
SCH3 for SR, we compare four candidate structures: two from others and two we
obtained. Our models are distinct from others in that thiolate groups form
monomers and dimers of the staple motif (a nearly linear RS-Au-SR bonding
unit). We examine the energetics and electronic structures of the four
structures with density functional theory (DFT) and compute their optical
spectra with time-dependent DFT. We show that our dimer-dominated model is over
2.6 eV lower in energy than the two models from others and 1.3 eV lower than
our previous monomer-dominated model. The dimer-dominated model also presents
good agreement with experiment for optical absorption.
