Correlated band structure of NiO, CoO, and MnO by variational cluster approximation

The variational cluster approximation proposed by Potthoff is applied to the calculation of the single-particle spectral function of the transition-metal oxides MnO, CoO, and NiO. Trial self-energies and the numerical value of the Luttinger-Ward functional are obtained by exact diagonalization of a TMO₆ cluster. The single-particle parameters of this cluster serve as variational parameters to construct a stationary point of the grand potential of the lattice system. The stationary point is found by a crossover procedure, which allows to go continuously from an array of disconnected clusters to the lattice system. The self-energy is found to contain irrelevant degrees of freedom, which have marginal impact on the grand potential and need to be excluded to obtain meaningful results. The obtained spectral functions are in good agreement with experimental data.