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