  Recent work has shown that a finite harmonic oscillator basis in nuclear
many-body calculations effectively imposes a hard-wall boundary condition in
coordinate space, motivating infrared extrapolation formulas for the energy and
other observables. Here we further refine these formulas by studying two-body
models and the deuteron. We accurately determine the box size as a function of
the model space parameters, and compute scattering phase shifts in the harmonic
oscillator basis. We show that the energy shift can be well approximated in
terms of the asymptotic normalization coefficient and the bound-state momentum,
discuss higher-order corrections for weakly bound systems, and illustrate this
universal property using unitarily equivalent calculations of the deuteron.
