  We have analyzed available optical data for Au in the mid-infrared range
which is important for a precise prediction of the Casimir force. Significant
variation of the data demonstrates genuine sample dependence of the dielectric
function. We demonstrate that the Casimir force is largely determined by the
material properties in the low frequency domain and argue that therefore the
precise values of the Drude parameters are crucial for an accurate evaluation
of the force. These parameters can be estimated by two different methods,
either by fitting real and imaginary parts of the dielectric function at low
frequencies, or via a Kramers-Kronig analysis based on the imaginary part of
the dielectric function in the extended frequency range. Both methods lead to
very similar results. We show that the variation of the Casimir force
calculated with the use of different optical data can be as large as 5% and at
any rate cannot be ignored. To have a reliable prediction of the force with a
precision of 1%, one has to measure the optical properties of metallic films
used for the force measurement.
