Remark on the computation of mode sums - art. no. 066006

The computation of mode sums of the types encountered in basic quantum fiel d theoretic applications is addressed with an emphasis on their expansions into functions of distance that can be interpreted as potentials. We show h ow to regularize and calculate the Casimir energy for the continuum Nambu-G oto string with massive ends as well as for the discrete Isgur-Paton non-re lativistic string with massive ends. As an additional example, we examine t he effect on the interquark potential of a constant Kalb-Ramond field stren gth interacting with a QCD string.