Model of the host structure

The adsorption process is simulated in a finite unit cell using periodic boundary conditions within the $\Gamma$$-$point approximation (see below). As a result, only the wavelengths corresponding to integer fractions of the unit cell extent are admitted in the solutions. In principle, by using larger and larger unit cells, the error could be decreased at will. In practice, for any useful unit cell size, the $\Gamma$$-$point approximation is a severe one. This approach, however, cancels most of the error by the counterpoise correction: The ration is calculated as partition functions of the same box, which is first subject to the host potential and then potential free partition functions of H2. In both cases, the box has the same geometry and implies the same nodal behaviour to the adsorbed and free wave functions.