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We study the geometry dependence of the Casimir energy for
deformed metal plates by a path integral quantization of the
electromagnetic field. For the first time, we give a complete
analytical result for the deformation induced change in Casimir
energy $\delta\varepsilon$ in an experimentally testable,
nontrivial geometry, consisting of a flat and a corrugated plate.
Our results show an interesting crossover for $\delta\varepsilon$
as a function of the ratio of the mean plate distance \emph{H}, to
the corrugation length $\lambda:$ For $\lambda<< H $ we find a
slower decay $\sim H^{-4}$, compared to the $H^{-5}$ behavior
predicted by the commonly used pairwise summation of van der Waals
forces, which is valid only for$\lambda\gg H$.
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