The Si(557)-Au surface, containing monatomic Au wires parallel to the steps, has been proposed as an experimental realization of an ideal one-dimensional metal. In fact, recent photoemission experiments on this system (Nature 402, 504 (1999)) found two peaks that were interpreted in terms of the spin-charge separation in a Luttinger liquid. Our first-principles density functional calculations reveal two metallic bands associated with Au-Si bonds, instead of the single band expected from the Au 6s states, providing an alternative explanation for the experimental observations. PACS Numbers: 73.20.At, 71.15.Mb, 71.10.Pm, 68.35.Bs It has been recognized for several decades now that a one-dimensional metal should considerably deviate from the Fermi liquid behavior [1][2][3][4]: quasiparticles would be replaced by two distinct collective excitations involving spin and charge, and some of the system's properties would exhibit characteristic power-law dependences. Despite this theoretical understanding, the experimental observation of the so-called Luttinger liquid behavior has proven quite elusive, due in large part to the lack of wellcharacterized one-dimensional systems. Only very recently, signatures of Luttinger liquid behavior have been observed by means of transport measurements in carbon nanotubes [5], in edge states of the fractional quantum Hall effect [6], and in quantum wires with a single conducting channel [7], and by optical conductivity measurements in organic conductors [8], and in the quasi-onedimensional CuO chains in PrBa 2 Cu 3 O 8 [9].More direct evidence of the Tomonaga-Luttinger behavior can be provided by angle-resolved photoemission experiments, in which the spin-charge splitting of the low-energy excitations can, in principle, be measured. This has been tried for several systems containing quasione-dimensional metallic chains [10,11]. However, in most cases the results were inconclusive due to the lack of energy and angular resolution [12], the difficulties to correctly align the chains, and other uncertainties associated with the crossover of dimensionality in the bulk samples. To avoid some of these problems, Segovia et al.[13] studied the photoemission from a monophase sample of the stepped Si(557)-Au surface, in which the gold atoms are believed to form single rows running parallel to the edge of each step. For this system, the measured spectra were assigned to a single one-dimensional band derived from the gold 6s states. The observed splitting of this band close to the Fermi level was interpreted in terms of the spin-charge separation in a one-dimensional electron liquid, providing the first direct measurement of two distinct peaks for spinons and holons. If such behavior is intrinsic to Au wires, it could also apply to monatomic gold wires fabricated using a scanning tunneling microscope (STM) and to mechanically controllable break junctions [14,15].In spite of this extraordinary observation, very little is still known about the atomic and the electronic structure of the Si(557)-Au s...