  We have conducted a series of numerical experiments using spherically
symmetric, general relativistic, neutrino radiation hydrodynamics with the code
Agile-BOLTZTRAN to examine the effects of modern neutrino opacities on the
development of supernova simulations. We test the effects of opacities by
removing opacities or by undoing opacity improvements for individual opacities
and groups of opacities. We find that improvements to electron capture (EC) on
nuclei, namely EC on an ensemble of nuclei using modern nuclear structure
models rather than the simpler independent-particle approximation (IPA) for EC
on a mean nucleus, plays the most important role during core collapse of all
tested neutrino opacities. Low-energy neutrinos emitted by modern nuclear EC
preferentially escape during collapse without the energy downscattering on
electrons required to enhance neutrino escape and deleptonization for the
models with IPA nuclear EC. During shock breakout the primary influence on the
emergent neutrinos arises from NIS on electrons. For the accretion phase,
non-isoenergetic scattering on free nucleons and pair emission by $e^+e^-$
annihilation have the largest impact on the neutrino emission and shock
evolution. Other opacities evaluated, including nucleon--nucleon bremsstrahlung
and especially neutrino--positron scattering, have little measurable impact on
neutrino emission or shock dynamics. Modern treatments of nuclear electron
capture, $e^+e^-$-annihilation pair emission, and non-isoenergetic scattering
on electrons and free nucleons are critical elements of core-collapse
simulations of all dimensionality.
