The adsorption and diffusion of cross-channel ad-dimers and ad-trimers was investigated using ab-initio DFT calculations. In contrast to in-channel dimers, the motion of cross-channel dimers proceeds one ad-atom at the time with an activation energy comparable to observed for quasi-isolated ad-atoms. The separation of ad-atoms by the surface channel wall lowers, but not eliminates, ad-atoms interactions. Pair interactions show long-range oscillatory behavior with an electronic origin. The short range interactions are modified by creation of straight and staggered direct bonds. Motion of ad-trimers proceeds in the same fashion as for cross-channel ad-dimers. (C) 2014 Elsevier B.V. All rights reserved.