High-order commensurate structures of $CF4$ on Cu(110) from interaction potential calculations

Potential energy calculations are performed in order to interpret the high-order commensurate $(13×18)$ and $(2×18)$ structures of the $CF4$ layer adsorbed on Cu(110) determined from helium diffraction experiments. We find that the most stable geometry at $0$ K is a low order commensurate $(2×2)$ phase containing two $CF4$ molecules which are dipod oriented in the Cu troughs. However, several other configurations close to this stable structure with tilted dipod and tripod orientations yield total binding energies per molecule which are only 10 meV (i.e., less than 5%) weaker. Due to the lack of additional information on the potential accuracy, we find that the minimization procedure has difficulty discriminating unequivocally between structures with a large number of molecules per unit cell and for which changes in molecular orientations do not sensitively modify the total energy in the cell. In order to recover the experimental structure we propose a $(2×18)$ phase with 18 $CF4$ molecules per unit cell, deduced from the optimization calculation and leading to an electronic corrugation above the surface which is compatible with the measured helium diffraction profile.