  Difference between bulk material and deposited film is shown to have an
appreciable influence on the Casimir force. Analysis of the optical data on
gold films unambiguously demonstrates the sample dependence: the dielectric
functions of the films deposited in different conditions are different on the
level that cannot be ignored in high precision prediction of the force. It is
argued that the precise values of the Drude parameters are crucial for accurate
evaluation of the force. The dielectric function of perfect single crystalline
gold is discussed. It is used to establish the upper limit on the absolute
value of the force. It is demonstrated that the force between films is smaller
than that between bulk samples mainly due to the presence of voids and electron
scattering on the grain boundaries in the films. The minimal reduction of the
force is estimated as 2% for the smallest separations investigated in the most
precise experiments. The other sample effects can reduce the force further but
the correction is expected to be smaller than 1%.
