Systematic expansion for infrared oscillator basis extrapolations

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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 Eₗ 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 Eₗ 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.