Ab initio calculation of correlation effects for the O 1s core electron binding energy in MgO.

Wavefunction based ab initio embedded cluster calculations are employed to calculate the O 1s core electron binding energies (CEBEs) of bulk MgO and the MgO(001) surface. A quantum cluster consisting of 61 atoms in five layers and embedded in a large point charge field is used for bulk MgO, the cluster for the MgO(001) surface is chosen accordingly. The O 1s CEBEs are calculated at the Koopmans' theorem (KT) and ΔSCF levels and with inclusion of correlation effects by means of the MC-CEPA method (multi-configuration coupled electron pair approximation), which is an approximate multi-reference coupled cluster approach. The correlation contributions to the O 1s CEBE of the central O atom due to the Mg atoms in the first and the O atoms in the second coordination shell turned out to be additive to a large extent. Therefore, they could be evaluated in an incremental fashion by a series of smaller calculations, where only a few atoms are included in the correlation treatment rather than all atoms of the first coordination shells or of the full quantum cluster. This makes the calculations feasible, even if large basis sets are used. The final results for the O 1s CEBEs are 533.47 and 533.50 eV for bulk MgO and the MgO(001) surface, to which electron correlation contributes 0.77 and 0.70 eV, respectively.