Warm Dense Matter (WDM) represents a intermediate state of matter between condensed matter and ideal plasmas, serving as a critical research subject in fields such as inertial confinement fusion (ICF), laboratory astrophysics, geophysics, and planetary physics. Theoretical simulations of WDM absorption spectra (i.e., frequency-dependent radiative opacity) are pivotal in ICF research. They are not only directly relevant to the state diagnostics of the ablator materials and main fuel of targets but also constitute core parameters for radiation hydrodynamics simulations.
In this work, based on the physical picture of optical absorption, we re-derived the expression for the absorption cross-section within the framework of quantum perturbation theory. The derivation demonstrates that intraband-transition effects also make significant contributions to the absorption characteristics of WDM. Building upon this expression, we propose a simplified first-principles simulation method to calculate intraband transition intensities in absorption spectra and apply it to compute the absorption coefficients of dense aluminum plasmas.
Our calculations reveal a substantial discrepancy between the number of effective electrons influencing the optical and transport properties of WDM and the number of free electrons typically derived from band-structure statistics. Further analysis indicates that this difference stems from the increase in electron effective mass induced by the potential field in the dense plasma environment. These findings hold significant potential value for advancing high-precision theoretical modeling of radiative opacity in ICF, planetary, and stellar physics.

Warm dense matter;,Aluminum,First-principles calculation,Radiation Opacity,Absorption Spectrum