The title material has a quasi-one-dimensional electronic structure and is of considerable interest because it has a metallic phase with properties different from a simple Fermi liquid, a poorly understood "insulating" phase, and a superconducting phase which may involve spin triplet Cooper pairs. Using the Slater-Koster approach and comparison with published band-structure calculations we present the simplest possible tight-binding model for the electronic band structure near the Fermi energy. This describes a set of ladders with weak (and frustrated) interladder hopping. In the corresponding lattice model the system is actually close to one-quarter filling (i.e., one electron per pair of sites) rather than half-filling, as has often been claimed. We consider the simplest possible effective Hamiltonian that may capture the subtle competition between unconventional superconducting, charge ordered, and non-Fermi liquid metal phases. We argue that this is an extended Hubbard model with long-range Coulomb interactions. Estimates of the relevant values of the parameters in the Hamiltonian are given. Nuclear magnetic resonance relaxation rate experiments should be performed to clarify the role of charge fluctuations in Li0.9Mo6O17 associated with the proximity to a Coulomb driven charge ordering transition.
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