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The vacuum expectation value of the stress energy tensor for a massive
scalar field with arbitrary coupling in flat spaces with non-trivial
topology is discussed. We calculate the Casimir energy in these manifolds
employing the recently proposed optical approach based on closed classical
paths. The evaluation of the Casimir energy consists in an expansion in
terms of the lengths of these paths. We will show how different paths and
weight factors contribute in the calculation. We also find the mass and
temperature dependance for a pair of parallel conductors and show that the
massive fields cannot be neglected in high and low temperature regimes.
The optical approach is applied to twisted as well as spinor fields and
the results obtained are compared with those in the literature.
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