  Two-body matrix elements of arbitrary local interactions are written as the
sum of separable terms in a way that is well suited for the exchange and
pairing channels present in mean-field calculations. The expansion relies on
the transformation to center of mass and relative coordinate (in the spirit of
Talmi's method) and therefore it is only useful (finite number of expansion
terms) for harmonic oscillator single particle states. The converge of the
expansion with the number of terms retained is studied for a Gaussian two body
interaction. The limit of a contact (delta) force is also considered. Ways to
handle the general case are also discussed.
