The weakly bound CO molecule adsorbed on the low-index CeO2 surfaces: A case for a CCSD(T) benchmark study using an embedded-cluster model

The binding energy and the vibrational stretching frequency of the probe molecule CO adsorbed on the low-index CeO2 surfaces [(100), (110), and (111)] were benchmarked using the coupled-cluster singles, doubles, perturbative triples [CCSD(T)] method, employing an embedded cluster approach. Using the same methodology as for the top configuration of CO on the (111) surface [J. Vázquez Quesada et al., J. Chem. Phys. 161, 224707 (2024)], the best theoretical estimate for the CO frequency on the CeO2(100) surface (CO bridge configuration) obtained at the CCSD(T)/def2-TZ/QZVPP level of theory and under low-coverage conditions (2193 cm-1) is 17 cm-1 larger than the experimental value (1 ML coverage saturation), which is in agreement with previous estimates for the CO adsorption on the CeO2(111) surface (12 cm-1). For the (110) surface, theoretical and experimental data compare differently. The CCSD(T)/def2-TZ/QZVPP values are -7 cm-1 (top configuration) and -21 cm-1 (tilt-x configuration) lower than the two experimental features measured at 2170 cm-1 (negative feature) and 2160 cm-1 (positive feature). MP2 predictions suggest the existence of a case of multiple-configuration dynamics with various almost isoenergetic configurations in a low-coverage situation. The CO harmonic vibrational frequencies were not semi-empirically scaled but explicitly corrected for anharmonic effects, which amount to 25-26 cm-1 with all tested methods. CO adsorption energies of -0.40 ± 0.07 eV, -0.17 ± 0.07 eV, and -0.20 ± 0.07 eV for the (100), (110) (top), and (110) (tilt-x) adsorption sites, respectively, are obtained at the CCSD(T)/def2-TZ/QZVPP level of theory. These results agree well with those proposed for the (111) surface (-0.22 ± 0.07 eV) [J. Vázquez Quesada et al., J. Chem. Phys. 161, 224707 (2024)] and confirm the physisorption character of the adsorption of CO on the three low-index surfaces of CeO2.