On the use of constrained density-functional theory in determining model Hamiltonians. Test calculations for the H-2 molecule and a C chain

摘要

Constrained density-functional theory (CDFT) has been used by several authors for determining model Hamiltonian parameters of high-T-c superconductors and other transition metal compounds. These methods use the fact that the orbitals are well localized and can easily be split into atomic components. We generalize them to systems with relatively delocalized electrons, so that they can be used, for example, for determining parameters for a Hubbard model of conducting polymers (e.g., polyacetylene). We selected the H-2 molecule and a linear C chain as test cases. The results of the constrained density-functional calculations were fined to Hartree-Fock model calculations of the Hubbard model. In the case of H-2 we also fitted the CDFT results to exact solutions of the Hubbard model. For H-2 we found a substantial discrepancy between the U-values from the exact and the Hartree-Fock model calculations, which shows that it is important to specify the approximations that are used in determining the model parameters. On the other hand, the CDFT calculations themselves lead to roughly uniquely defined model parameters. [References: 12]