Stolze, JoachimValentí, RoserHirschfeld, Peter J.2008-06-032008-06-031991Valenti, Roser; Stolze, Joachim; Hirschfeld, Peter J.: Lower Bounds for the Ground-State Energies of the 2D Hubbard and t-J Models. In: Physical Review B Nr. 16, Jg. 43(1991), S. 13743-13746, doi: 10.1103/PhysRevB.43.13743 .http://hdl.handle.net/2003/2542710.17877/DE290R-3489We present simple lower bounds on the ground-state energy of the two-dimensional (2D) Hubbard and t-J models for arbitrary values of band filling and coupling constant. For the Hubbard model we derive two types of bounds, both based on decomposing the model Hamiltonian into a sum of sub-Hamiltonians. For a decomposition into local cluster sub-Hamiltonians, we perform a generalized Legendre transform on previously derived bounds for the grand-canonical potential. For a decomposition into spin-up and spin-down parts, previous results on the spinless Falicov-Kimball model may be used to obtain bounds for the Hubbard model, generalizing a result of Langer and Mattis to arbitrary filling. For the 2D t-J model we have only considered the decomposition into clusters. The 1D Hubbard model is used as a test case. The bounds may be improved by diagonalizing the Hamiltonian for larger clusters.enThe American Physical Society51010.1103/PhysRevB.43.13743article (journal)