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 vibrational stretching frequency of the probe molecule CO adsorbed on the low-index CeO2 surfaces [(100), (110), and (111)] was benchmarked using the CCSD(T) method and employing an embedded cluster approach. Using the same methodology as for the (111) surface [J. Chem. Phys., 161 (2024) 224707] the best theoretical estimate for the CO frequency on the CeO2(100) surface obtained at the CCSD(T)/def2-TZ/QZVPP level of theory and under low-coverage conditions (2193 cm−1) results to be 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) and -16 cm−1 (tilt-x) 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 configurations almost isoenergetic 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 {plus minus}0.07 eV, - 0.17 eV {plus minus} 0.07 eV, and -0.20 eV {plus minus} 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 with those proposed for the (111) surface (-0.22 eV {plus minus} 0.07 eV) and confirms the physisorption character of the adsorption of CO on the three low-index surfaces of CeO2.