  Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon
energy, 5 fs duration, at the $\sim 19-25$ eV/atom absorbed doses, is studied
theoretically on its way to warm dense matter state. Simulations with our
hybrid code XTANT show disordering on sub-100 fs timescale, with the
diffraction peak (220) vanishing faster than the peak (111). The warm dense
matter formation proceeds as a nonthermal damage of diamond with the band gap
collapse triggering atomic disordering. Short-living graphite-like state is
identified during a few femtoseconds between the disappearance of (220) peak
and the disappearance of (111) peak. The results obtained are compared with the
data from the recent experiment at SACLA, showing qualitative agreement.
Challenges remaining for the accurate modeling of the transition of solids to
warm dense matter state and proposals for supplementary measurements are
discussed in detail.
