OpenMC/tests/regression_tests/density/materials.xml

<?xml version="1.0"?>
<materials>

  <material id="1">
    <density value="0.1" units="atom/b-cm" />
    <nuclide name="U235" ao="1.0" />
  </material>

  <material id="2">
    <density value="4.5e22" units="atom/cm3" />
    <nuclide name="U235" ao="1.0" />
  </material>

  <material id="3">
    <density value="12.3e3" units="kg/m3" />
    <nuclide name="U235" ao="1.0" />
  </material>

  <material id="4">
    <density units="sum" />
    <nuclide name="U235" ao="0.3e-2" />
    <nuclide name="U238" ao="0.5e-1" />
    <nuclide name="H1" ao="0.1e-2" />
  </material>

</materials>