  Recent work has demonstrated that the infrared effects of harmonic oscillator
basis truncations are well approximated by imposing a partial-wave Dirichlet
boundary condition at a properly identified radius L. This led to formulas for
extrapolating the corresponding energy E_L and other observables to infinite L
and thus infinite basis size. Here we reconsider the energy for a two-body
system with a Dirichlet boundary condition at L to identify and test a
consistent and systematic expansion for E_L that depends only on observables.
We also generalize the energy extrapolation formula to nonzero angular
momentum, and apply it to the deuteron. Formulas given previously for
extrapolating the radius are derived in detail.
