The effect of intense ultrashort irradiation on interatomic forces, crystal stability, and possible melting transition of the underlying lattice is not completely elucidated. By using ab initio linear response to compute the phonon spectrum of gold, silicon, and aluminum, we found that silicon and gold behave in opposite ways when increasing radiation intensity: whereas a weakening of the silicon bond induces a lattice instability, gold undergoes a sharp increase of its melting temperature, while a significantly smaller effect is observed for aluminum for electronic temperatures up to 6 eV.

It is shown that a modified scheme of density functional theory, using the Thomas-Fermi kinetic energy functional for the electrons, is well suited to perform very-high-temperature molecular dynamics simulations on high-Z elements. As an example, iron on the principal Hugoniot is simulated up to 5 keV and 5 times the normal density, giving an equation of state in agreement with current models. Ionic structure is obtained and is given to an excellent level of precision by the structure of the one-component plasma computed for a coupling parameter corresponding to Thomas-Fermi ionization.

Ab initio molecular dynamics is used to compute the thermal conductivity of hydrogen at 80 g cm(-3) and temperature up to 800 eV. Pressures and ionic structure are compared with orbital-free calculations. Thermal conductivity is evaluated using the Kubo-Greenwood formula and is compared with models currently used in hydrodynamical simulations of inertial confinement fusion.