The eigenvalues of a Hubbard Hamiltonian for a four-center tetrahedral cluster are calculated exactly. Full use is made of the symmetry of the problem, which is analyzed for an arbitary number of electrons, $0\le N\le 8$. Comparison is made with the phenomenological Hund's-rule predictions for the ground states. The diversity of the low-energy states is surprising: magnetic and nonmagnetic solutions, single and degenerate representations, accidental degeneracies, and symmetry crossovers are all found for the ground states. Implications for three-dimensional lattices are discussed.

• Received 29 March 1984

DOI:https://doi.org/10.1103/PhysRevB.30.1695