In the case of a (2 × 2) unit cell, the barrier is the same, but the minimum energy path is significantly longer as shown in Figure . While for larger unit cells all H atoms but the two desorbing ones remain still close to their equilibrium fcc sites, in the case of (2 × 2) unit cell, the third H atom that is closest to the desorbing atoms moves from fcc to hcp site. This means that the configurational space leading to desorption is significantly different in the case of the (2 × 2) unit cell. Note that this is the cell that was used for all previous analytical potential constructions. 7,10 Below, we discuss the influence it has on the results of laser-induced desorption simulations.