  Kramers-Kronig analysis is commonly used to estimate the optical properties
of new materials. The analysis typically uses data from far infrared through
near ultraviolet (say, 40--40,000 cm$^{-1}$ or 5 mev--5 eV) and uses
extrapolations outside the measured range. Most high-frequency extrapolations
use a power law, 1/$\omega^n$, transitioning to $1/\omega^{4}$ at a
considerably higher frequency and continuing this free-carrier extension to
infinity. The mid-range power law is adjusted to match the slope of the data
and to give pleasing curves, but the choice of power (usually between 0.5 and
3) is arbitrary. Instead of an arbitrary power law, it is is better to use
X-ray atomic scattering functions such as those presented by Henke and
co-workers. These basically treat the solid as a linear combinations of its
atomic constituents and, knowing the chemical formula and the density, allow
the computation of dielectric function, reflectivity, and other optical
functions. The "Henke reflectivity" can be used over photon energies of 10
eV--34 keV, after which a $1/\omega^{4}$ continuation is perfectly fine. The
bridge between experimental data and the Henke reflectivity as well as two
corrections that needed to be made to the latter are discussed.
