  A key requirement for the correct interpretation of high-resolution X-ray
spectra is that transition energies are known with high accuracy and precision.
We investigate the K-shell features of Ne, CO$_2$, and SF$_6$ gases, by
measuring their photo ion-yield spectra at the BESSY II synchrotron facility
simultaneously with the 1s-np fluorescence emission of He-like ions produced in
the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions
provide the basis of the calibration. While the CO$_2$ result agrees well with
previous measurements, the SF$_6$ spectrum appears shifted by ~0.5 eV, about
twice the uncertainty of the earlier results. Our result for Ne shows a large
departure from earlier results, but may suffer from larger systematic effects
than our other measurements. The molecular spectra agree well with our results
of time-dependent density functional theory. We find that the statistical
uncertainty allows calibrations in the desired range of 1-10 meV, however,
systematic contributions still limit the uncertainty to ~40-100 meV, mainly due
to the temporal stability of the monochromator energy scale. Combining our
absolute calibration technique with a relative energy calibration technique
such as photoelectron energy spectroscopy will be necessary to realize its full
potential of achieving uncertainties as low as 1-10 meV.
