Constrained density-functional calculations for delocalized electrons: Hubbard parameters for polyacetylene

The concept of constrained density-functional calculations is extended to situations with sets of non-orthogonal basis functions that are better than minimal and used for systems with delocalized electrons. The resulting method is subsequently applied in calculating single- and many-body parameters for extended Hubbard models for the trans, cis-trans, and trans-cis isomers of polyacetylene within the Hartree–Fock approximation. It is found that the on-site Hubbard U is large, about 10 eV, whereas the nearest-neighbour parameter V is very small, and that the nearest-neighbour hopping integrals depend strongly on the system.