The density which is provided by the user via the WHAT(3) of a MATERIAL
card is used for transport, i.e. it is used to convert cross sections to
mean free paths. Therefore, it must be the actual density in case of a gas
at non-atmospheric pressure, or the average density in case of a non
uniform material. However, it is not sufficient because:

a) if the material is a gas, the Sternheimer-Peierls formula must be
   evaluated first at NTP and then corrected for actual TP. The density at
   NTP used in the evaluation of the formula is derived as
   MATERIAL-WHAT(3)/MAT-PROP-WHAT(1)
b) the material density can be non-uniform (for instance a material with
   voids (like a sponge or a powder).
   Transport (mean free path) is evaluated based on the average density
   MATERIAL-WHAT(3), while dE/dx must be evaluated based on local density
   MATERIAL-WHAT(3)/MAT-PROP-WHAT(2).