  Dispersion relations for dipolar modes propagating along a chain of metal
nanoparticles are calculated by solving the full Maxwell equations, including
radiation damping. The nanoparticles are treated as point dipoles, which means
the results are valid only for a/d <= 1/3, where a is the particle radius and d
the spacing. The discrete modes for a finite chain are first calculated, then
these are mapped onto the dispersion relations appropriate for the infinite
chain. Computed results are given for a chain of 50-nm diameter Ag spheres
spaced by 75 nm. We find large deviations from previous quasistatic results:
Transverse modes interact strongly with the light line. Longitudinal modes
develop a bandwidth more than twice as large, resulting in a group velocity
that is more than doubled. All modes for which k_mode <= w/c show strongly
enhanced decay due to radiation damping.
