  The free-free opacity in dense systems is a property that both tests our
fundamental understanding of correlated many-body systems, and is needed to
understand the radiative properties of high energy-density plasmas. Despite its
importance, predictive calculations of the free-free opacity remain challenging
even in the condensed matter phase for simple metals. Here we show how the
free-free opacity can be modelled at finite-temperatures via time-dependent
density functional theory, and illustrate the importance of including local
field corrections, core polarization and self-energy corrections. Our
calculations for ground-state Al are shown to agree well with experimental
opacity measurements performed on the Artemis laser facility across a wide
range of x-ray to ultraviolet wavelengths. We extend our calculations across
the melt to the warm-dense matter regime, and find good agreement with advanced
plasma models based on inverse bremsstrahlung at temperatures above 10 eV.
