Casimir Energies on a Twisted Two-Torus

We consider a twisted massless multiplet on a two-torus, one side with the normal boundary and the other with twisted boundary. The Casimir energy is calculated and regularized by means of the Epstein-Hurwitz type zeta function introduced by Elizalde. The resulted dimensions of spacetime for the twisted case may be integers. The results are compared with those of untwisted case. Since twisted Casimir energy is lower than untwisted energy, the untwisted cases may change into the twisted state in the spacetime.