  The shape of the OB-star spectral energy distribution is a critical component
in many diagnostics of the ISM and galaxy properties. We use single-star HII
regions from the LMC to quantitatively examine the ionizing SEDs from widely
available CoStar, TLUSTY, and WM-basic atmosphere grids. We evaluate the
stellar atmosphere models by matching the emission-line spectra that they
predict from CLOUDY photoionization simulations with those observed from the
nebulae. The atmosphere models are able to reproduce the observed optical
nebular line ratios, except at the highest energy transitions > 40 eV, assuming
that the gas distribution is non-uniform. Overall we find that simulations
using WM-basic produce the best agreement with the observed line ratios. The
rate of ionizing photons produced by the model SEDs is consistent with the rate
derived from the \Halpha\ luminosity for standard, log(g) = 4.0 models adopted
from the atmosphere grids. However, there is a systematic offset between the
rate of ionizing photons from different atmosphere models that is correlated
with the relative hardness of the SEDs. In general WM-basic and TLUSTY
atmosphere models predict similar effective temperatures, while CoStar predicts
effective temperatures that are cooler by a few thousand degrees. We compare
our effective temperatures, which depend on the nebular ionization balance, to
conventional photospheric-based calibrations from the literature. We suggest
that in the future, spectral type to effective temperature calibrations can be
constructed from nebular data.
