 The increasing of "Cv" from He to Xe for all metal substrates is caused by the larger atomic polarizability of the heavier rare gas atoms.For the position of the dynamical image plane, it decreases with increasing dielectric function and is typically on the order of 0.2 Å.Even though the van der Waals interaction is attractive, as the adsorbed atom moves closer to the surface the wavefunction of electron starts to overlap with that of the surface atoms.