  It has been known that the fundamental Lifshitz theory, which is based on
first principles of thermal quantum field theory, experiences difficulties when
compared with precise measurements of the Casimir force. We analyze the
nonconventional fit of response functions of many materials along the imaginary
frequency axis to the empirical model of "modified" oscillators which was
recently proposed in the literature. According to our results, this model is
unacceptable because at high frequencies it leads to the asymptotic behavior of
response functions which is in contradiction with that following from the
fundamental physical principles. We calculate the Casimir interaction in the
configurations of several precise experiments using the Lifshitz theory and the
response functions to quantized electromagnetic field expressed in terms of
modified oscillators and demonstrate that the obtained results are excluded by
the measurement data. This invalidates a claim made in the literature that the
Casimir-van der Waals forces calculated using these response functions are in
remarkable agreement with the experimental values. Possible reasons for a
disagreement between experiment and theory are discussed, and the way to
improve the situation is directed.
