Fermi Surface Nesting and the Origin of the Charge Density Wave in NbSe₂

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We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, Re chi₀(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi₀(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi₀(q) near the CDW wave vector, but that no such peaks are visible in Im chi₀(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi₀(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electron-electron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.