  Dark matter detectors require calibrations of their energy scale and
efficiency to detect nuclear recoils in the $1-50$ keV range. Most calibrations
use neutron scattering and require MCNP or Geant4 simulations of neutron
propagation through the detector. For most nuclei heavier than $^{16}$O, these
simulations' libraries ignore the contribution of resolved resonances to the
neutron elastic differential cross-section. For many isotopes and neutron
energies of importance to dark matter detection, this invalid assumption can
severely distort simulated nuclear recoil spectra. The correct angular
distributions can be calculated from the resonance parameters using R-matrix
formalism. A set of neutron scattering libraries with high resolution angular
distributions for MCNP and Geant4 of $^{19}$F, $^{40}$Ar, $^{50,52}$Cr,
$^{56}$Fe, $^{136}$Xe, and $^{206,207,208}$Pb is presented. An MCNPX library
for simulating the production of low-energy neutrons in the
$^9$Be$(\gamma,n)^8$Be reaction is also presented. Example dark matter detector
calibrations are simulated with the new libraries showing how detector
sensitivity could be overestimated by factors of two by relying on existing
MCNP and Geant4 libraries.
