CALCULATED HUBBARD INTERACTION IN THE NEUTRAL MX CHAIN SYSTEMS

We have performed first-principles local-density-approximation (LDA) e lectronic-structure calculations of the of the neutral MX chain compou nds and fitted their band structures to a two-band tight-binding Hamil tonian. Using two slightly different approaches based on constrained o ccupancies in the Kohn-Sham LDA, we have calculated from first princip les the Hubbard parameters for the neutral halogen-bridged transition- metal (MX) linear chain systems (M = Ni or Pt, and X = Cl, Br, or I). For PtX (X = Cl, Br) the Pt onsite U is almost three times larger than the Pt-X hopping integral, and is almost twice as large for the NiX c ompounds, which are very strongly correlated and are likely to have a spin-density-wave or antiferromagnetic ground state.