For a ${\mathrm{Cu}}_4$${\mathrm{O}}_8$ cluster with periodic boundary conditions the single-particle excitation spectrum is calculated rigorously by a numerical method with use of a realistic parameter set. The spectrum shows dispersive character at low energies as well as little dispersion at high excitation energies in accordance with experiments using angle-resolved photoemission. The ${\mathit{d}}^8$ final state is identified and its energy is found to be close to its experimental one.

• Received 10 April 1990

DOI:https://doi.org/10.1103/PhysRevB.42.2562