  The configuration interaction (CI) method for calculating the exact
eigenstates of a quantum-mechanical few-body system is problematic when applied
to particles interacting through contact forces. In dimensions higher than one
the approach fails due to the pathology of the Dirac delta-potential, making it
impossible to reach convergence by gradually increasing the size of the Hilbert
space. However, this problem may be cured in a rather simple manner by
renormalizing the strength of the contact potential when diagonalizing in a
truncated Hilbert space. One hereby relies on the comparison of the CI results
to the two-body ground-state energy obtained by the exact solution of the
Schroedinger equation for a regularized contact interaction. We here discuss a
scheme that provides cutoff-independent few-body physical observables. The
method is applied to a few-body system of ultracold atoms confined by a
two-dimensional harmonic oscillator.
