  We test the efficacy of excited state mean field theory and its
excited-state-specific perturbation theory on the prediction of K-edge
positions and X-ray peak separations. We find that the mean field theory is
surprisingly accurate, even though it contains no accounting of differential
electron correlation effects. In the perturbation theory, we test multiple
core-valence separation schemes and find that, with the mean field theory
already so accurate, electron-counting biases in one popular separation scheme
become a dominant error when predicting K-edges. Happily, these appear to be
relatively easy to correct for, leading to a perturbation theory for K-edge
positions that is lower scaling and more accurate than coupled cluster theory
and competitive in accuracy with recent high-accuracy results from restricted
open-shell Kohn Sham theory. For peak separations, our preliminary data show
excited state mean field theory to be exceptionally accurate, but more
extensive testing will be needed to see how it and its perturbation theory
compare to coupled cluster peak separations more broadly.
