Two Particles in a Trap
  The Busch-formula relates the energy-spectrum of two point-like particles interacting in a 3-D isotropic Harmonic Oscillator trap to the free scattering phase-shifts of the particles. This formula is used to find an expression for the \it shift \rm in the spectrum from the unperturbed (non-interacting) spectrum rather than the spectrum itself. This shift is shown to be approximately $\Delta=-\delta(k)/\pi\times dE$, where $dE$ is the spacing between unperturbed energy levels. The resulting difference from the Busch-formula is typically 1/2% except for the lowest energy-state and small scattering length when it is 3%. It goes to zero when the scattering length $\rightarrow \pm \infty$.   The energy shift $\Delta$ is familiar from a relatedproblem, that of two particles in a spherical infinite square-well trap of radius $R$ in the limit $R\rightarrow \infty$. The approximation ishowever as large as 30% for finite values of $R$, a situation quite different from the Harmonic Oscillator case.   The square-well results for $R\rightarrow \infty$ led to the use ofin-medium (effective) interactions in nuclear matter calculations that were $\propto \Delta$ and known as the \it phase shift approximation \rm.Our results indicate that the validity of this approximation depends on the trapitself, a problem already discussed by DeWitt more than 50 years ago for acubical vs spherical trap. 
H. S. Kohler
nucl-th quant-ph