  We propose a method for the approximate computation of the Green function of
a scalar massless field Phi subjected to potential barriers of given size and
shape in spacetime. This technique is applied to the case of a 3D gaussian
ellipsoid-like barrier, placed on the axis between two pointlike sources of the
field. Instead of the Green function we compute its temporal integral, that
gives the static potential energy of the interaction of the two sources. Such
interaction takes place in part by tunneling of the quanta of Phi across the
barrier. We evaluate numerically the correction to the potential in dependence
on the size of the barrier and on the barrier-sources distance.
