  We discuss the problem of two particles interacting via short-range
interactions within a harmonic-oscillator trap. The interactions are organized
according to their number of derivatives and defined in truncated model spaces
made from a bound-state basis. Leading-order (LO) interactions are iterated to
all orders, while corrections are treated in perturbation theory. We show
explicitly that next-to-LO and next-to-next-to-LO interactions improve
convergence as the model space increases. In the large-model-space limit we
regain results from a pseudopotential. Arbitrary scattering lengths are
considered, as well as a generalization to include the non-vanishing range of
the interaction.
