This contribution discusses challenges in modeling of formation of the warm dense matter (WDM) state in solids exposed to femtosecond X-ray free-electron laser pulses. It is based upon our previously reported code XTANT (X-rayinduced Thermal And Nonthermal Transition; N. Medvedev et. al., 4open 1, 3, 2018), which combines tight binding (TB) molecular dynamics for atoms with Monte Carlo modeling of high-energy electrons and core-holes, and Boltzmann collision integrals for nonadiabatic electron-ion coupling. The current version of the code, XTANT-3, includes LCAO basis sets sp³, sp³s*, and sp³d⁵, and can operate with both orthogonal and nonorthogonal Hamiltonians. It includes the TB parameterizations by Goodwin et al., a transferrable version of Vogl’s et al. TB, NRL, and DFTB. Considering that other modules of the code are applicable to any chemical element, this makes XTANT-3 capable of treating a large variety of materials. In order to extend it to the WDM regime, a few limitations that must be overcome are discussed here: shortrange repulsion potential must be sufficiently strong; basis sets must span large enough energy space within the conduction band; dependence of the electronic scattering cross sections on the electronic and atomic temperatures and structure needs to be considered. Directions at solving these issues are outlined in this proceeding.

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Nikita Medvedev, "Modeling warm dense matter formation within tight binding approximation," Proc. SPIE 11035, Optics Damage and Materials Processing by EUV/X-ray Radiation VII, 110350Q (Presented at SPIE Optics + Optoelectronics: April 02, 2019; Published: 24 April 2019); https://doi.org/10.1117/12.2520805.