  The lattice dynamics of the metallic tin {\beta} and {\gamma} polymorphs has
been studied by a combination of diffuse scattering, inelastic x-ray scattering
and density functional perturbation theory. The non-symmorphic space group of
the {\beta}-tin structure results in unusual asymmetry of thermal diffuse
scattering. Strong resemblance of the diffuse scattering intensity distribution
in {\beta} and {\gamma}-tin were observed, reflecting the structural
relationship between the two phases and revealing the qualitative similarity of
the underlying electronic potential. The strong influence of the electron
subsystem on inter-ionic interactions creates anomalies in the phonon
dispersion relations. All observed features are described in great detail by
density functional perturbation theory for both {\beta}- and {\gamma}-tin at
arbitrary momentum transfers. The combined approach delivers thus a complete
picture of the lattice dynamics in harmonic description.
