In case of z 0 (ε LUMO ), only PyMe + does not follow the linear trend. Since the z 0 distance is determined at the ion center of mass, the elongated shape of the PyMe molecule and, consequently, the orbitals may cause overestimation of the z 0 value. No correlation is observed between the anion adsorption and ε HOMO (Figure ), while in case of E ads (ε LUMO ), there is a volcano-like dependence with a maximum at ε LUMO = 0.245 a.u. (Figure ). The strongest binding to the surface is observed for the RCOO -, OH -, and F -systems, while that for the PF 6 anion is the weakest. OH -and F -are out of the E ads (ε LUMO ) trend, with the adsorption energies for the above ions getting as high as -4.0 and -4.3 eV, respectively; the adsorption energy in these cases is close to that of a strong chemical bond with the Al atoms.