A photo-electric absorption from approximation to a warm absorber using Balucinska-Church and McCammon (ApJ 400, 699) cross-sections. Relative abundances are set by the abund command.
where 
(E) is the photo-electric cross-section (NOT including Thomson
scattering) and
par1 = equivalent hydrogen column (in units of 10atoms/cm
)
par2 = window energy (keV)