Zero-point energy of massless scalar fields in the presence of soft and   semihard boundaries in D dimensions
  The renormalized energy density of a massless scalar field defined in a D-dimensional flat spacetime is computed in the presence of "soft" and "semihard" boundaries, modeled by some smoothly increasing potential functions. The sign of the renormalized energy densities for these different confining situations is investigated. The dependence of this energy on $D$ for the cases of "hard" and "soft/semihard" boundaries are compared. 
F. Caruso, R. De Paola and N. F. Svaiter