Chlorine adsorption on Ru(0001) surface has been studied by a combined density functional theory (DFT) and quantitative low energy electron diffraction (IEED) approach. The (root 3 x root 3)R30 degrees-Cl phase with theta(a) =1/3 ML and chlorine sitting in fcc sites has been identified by DFT calculations as the most stable chlorine adsorbate structure on Ru(0001) with an adsorption energy of -220 kJ/mol. The atomic geometry of (root 3 x root 3)R30 degrees-Cl was determined by quantitative LEED. The achieved agreement between experimental and simulated LEED data is quantified by a Pendry factor of r(p)= 0.19 for a fcc adsorption site with a Cl-Ru bond length of 2.52 angstrom. At chlorine coverages beyond 1/3 ML LEED reveals diffuse diffraction rings, indicating a continuous compression of the hexagonal Cl overlayer with a preferred average Cl-Cl distance of 4.7 A in the (root 3 x root 3)R30 degrees-Cl, theta(a) = 1/3 ML phase towards 3.9 A at saturation coverage of 0.48 ML. (C) 2011 Elsevier B.V. All rights reserved.