The wetting layer of DO on Ru(000 1) has been reanalyzed with ab initio molecular dynamics and full-potential calculation. The work function change induced by DO overlayers clearly favors the picture of molecular wetting rather than partial dissociation and suggests a mixed structure consisting of both hydrogen-up and hydrogen-down bilayers. This is further supported by the vibrational spectra, where agreement is only found for the intact overlayers but not for a half-dissociated overlayer. The larger barrier for dissociation in comparison with that for desorption suggests that dissociation is kinetically forbidden at low temperatures. These findings resolve the current controversy between previous calculations and experiments. (C) 2004 Elsevier B.V. All rights reserved.