Quantum mechanical modeling of electronic excitations in metal oxides: Magnesia as a prototype

We compare embedded correlated wavefunction (ECW) approaches for predicting excited states within MgO as a prototypical metal oxide. The crystal is partitioned into a cluster treated with CW methods and a background described by various electrostatic or orbital-free-density-functional-theory (DFT)-based embedding potentials. The excited singlet and triplet states are found to be nearly degenerate and of charge-transfer type, consistent with experiment. Although the prediction of excitation energies by ECW theory with an electrostatic description of the background falls slightly short of more expensive Green's function methods, it is significantly improved over standard DFT or non-embedded CW methods.