Casimir pressure in a multilayer system with a fixed total length

We consider Casimir pressure on a slab in a configuration consisting of various dielectrics with planar symmetry. In such configurations, one usually calculates the Casimir pressure (force) on a particular slab assuming that lengths of all other slabs remain unchanged. Alternatively, one can consider a multilayer system with a fixed total length. With this restriction only, the length of each slab can eventually be changed under the Casimir pressure that will try to minimize the total Casimir energy of the system. Here we calculate the Casimir pressure on the slab in such a 'constrained' configuration and compare the results with the standard approach. It turns out that, by applying different boundary conditions, one can obtain significantly different Casimir pressures on the same object. In particular, when the thicknesses of the slab and surrounding layers are on the nanometer scale, the Casimir pressure on the slab can change from strongly squeezing in the case of fixed thicknesses of surrounding slabs to strongly relaxing in the case when only the length of the total system remains fixed.