  The ultrafast dynamics of ions in solids following intense femtosecond laser
excitation is governed by two fundamentally distinct yet interplaying effects.
On one hand, the significant generation of hot electron-hole pairs by the light
pulse alters the strength and nature of interatomic bonding, resulting in
nonthermal ionic motion. On the other hand, incoherent electron-ion collisions
drives an equilibration of the electrons and the ions, which reach a common
temperature on a picosecond timescale. This letter introduces, for the first
time, a unified theory that comprehensively accounts for both phenomena. Our
approach is versatile, applicable to both ab-initio and large-scale molecular
dynamics simulations, and leads to a generalization of the two-temperature
model-molecular-dynamics equations of motion. To illustrate the effectiveness
of our method, we apply it to describe the laser excitation of silicon thin
films. Our simulations reproduce the time evolution of the Bragg peaks in
excellent agreement with the experiments.
