Title: Characterizing Feshbach resonances in ultracold scattering calculations,
Abstract: We describe procedures for converging on and characterizing zero-energy
Feshbach resonances that appear in scattering lengths as a function of an
external field. The elastic procedure is appropriate for purely elastic
scattering, where the scattering length is real and displays a true pole. The
regularized scattering length (RSL) procedure is appropriate when there is weak
background inelasticity, so that the scattering length is complex and displays
an oscillation rather than a pole, but the resonant scattering length $a_{\rm
res}$ is close to real. The fully complex procedure is appropriate when there
is substantial background inelasticity and the real and complex parts of
$a_{\rm res}$ are required. We demonstrate these procedures for scattering of
ultracold $^{85}$Rb in various initial states. All of them can converge on and
provide full characterization of resonances, from initial guesses many
thousands of widths away, using scattering calculations at only about 10 values
of the external field.