  Glycine molecules deposited on Cu(100) surface give rise to an anisotropic
free-electron-like (FEL) electronic dispersion in its p(2$\times$4)
superstructure, as reported in recent experiments [Phys. Rev. Lett. {\bf 99},
216102 (2007); J. Am. Chem. Soc. {\bf 129}, 740 (2007)]. Using density
functional theory and exhaustively calculating sixteen possible structures, we
have determined the molecular arrangement that can give the experimentally
observed FEL behavior. Eight configurations, among the sixteen, were not
investigated before in the literature and one of them (denoted Str-3) is able
to provide the FEL behavior in excellent agreement with the experiments. In
addition, the particular configuration Str-3 satisfies other criteria of the
observed p(2$\times$4) superstructure, e.g. chirality and cleavable
orientation.
