abundances of @vennesetal11-1 and the Mn abundance of @melisetal11-1 to calculate the corresponding diffusion fluxes.

As a result we have to conclude that diffusion and a stratified abundance structure lead only to minor adjustments of the abundances that cannot explain the large discrepancy between optical and ultraviolet determinations for silicon. There is, however, an important caveat to this conclusion. Our diffusion calculations use only the surface gravity (and as a minor effect the temperature gradient for thermal diffusion) as driving force. [@chayer+dupuis10-1] have recently demonstrated that for silicon, radiative levitation can lead to a negative effective gravity and support the atoms in the outer layers of the atmosphere against diffusion. They only published detailed data for a DAZ model with 20000K and $\log g=8.00$, and in their model only abundances smaller than $\log\mathrm{[Si/H]}=-8.0$ are really supported, because the lines saturate at higher abundances, effectively reducing the radiative support. However, it is quite feasible that even if the atoms are not totally supported, the diffusion velocity would be smaller, changing the abundance gradient. The answer to this puzzle will have to await similar, detailed models for a variety of stellar parameters and heavy elements that can be tested against the large 