  In a recent paper [1] the Casimir energy was calculated for a massive dirac
field in (1+1) dimensional space-time in the presence of an inverse square well
potential and shown to be positive. It will be shown that this result violates
a key assumption of quantum field theory which is that the vacuum state is the
state of minimum energy. The reason for this discrepancy is examined and is
shown to be related to the way the charge density operator is defined. If the
charge density operator is defined using point splitting then an extra term
will be added to the charge which will result in the Casimir energy being
negative.
