  We provide improved atomic calculation of wavelengths, oscillator strengths,
and autoionization rates relevant to the $2\to 3$ inner-shell transitions of
Fe~VI--XVI, the so-called Fe~M-shell unresolved transition array (UTA). A
second order many-body perturbation theory is employed to obtain accurate
transition wavelengths, which are systematically larger than previous
theoretical results by 15--45~m{\AA}. For a few transitions of Fe~XVI and Fe~XV
where laboratory measurements exist, our new wavelengths are accurate to within
a few m{\AA}. Using these new calculations, the apparent discrepancy in the
velocities between the Fe~M-shell UTA and other highly ionized absorption lines
in the outflow of NGC 3783 disappears. The oscillator strengths in our new
calculation agree well with the previous theoretical data, while the new
autoionization rates are significantly larger, especially for lower charge
states. We attribute this discrepancy to the missing autoionization channels in
the previous calculation. The increased autoionization rates may slightly
affect the column density analysis of the Fe~M-shell UTA for sources with high
column density and very low turbulent broadening. The complete set of atomic
data is provided as an electronic table.
