Using DFT calculations, we investigate how the prior chemisorption of an H atom on graphite influences the adsorption properties of a second H atom at neighboring sites of the same carbon ring of the surface. We emphasize a few features concerning site effects on the energy and geometry of chemisorption of the two H atoms as well as on the puckering of the surface. The study of the behavior of the potential energy surface along or near the minimum energy path shows for the first time that the second H atom chemisorption at the para site is barrier-less. (c) 2006 Elsevier B.V. All rights reserved.