Fermi-surface nesting and the origin of the charge-density wave in NbSe2

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the noninteracting susceptibility, Re chi(0)(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi(0)(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi(0)(q) near the CDW wave vector, but that no such peaks are visible in Im chi(0)(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi(0)(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.