  We report an inelastic x-ray scattering investigation of phonons in FeSe
superconductor. Comparing the experimental phonon dispersion with density
functional theory (DFT) calculations in the non-magnetic state, we found a
significant disagreement between them. Improved overall agreement was obtained
by allowing for spin-polarization in the DFT calculations, despite the absence
of magnetic order in the experiment. This calculation gives a realistic
approximation, at DFT level, of the disordered paramagnetic state of FeSe, in
which strong spin fluctuations are present.
