  Laser melting of semiconductors has been observed for almost 40 years;
surprisingly, it is not well understood where most theoretical simulations show
a laser-induced thermal process. $\textit{Ab initio}$ nonadiabatic simulations
based on real-time time-dependent density functional theory reveal intrinsic
nonthermal melting of silicon, at a temperature far below the thermal melting
temperature of 1680 K. Both excitation threshold and time evolution of
diffraction intensity agree well with experiment. Nonthermal melting is
attributed to excitation-induced drastic changes in bonding electron density,
and the subsequent decrease in the melting barrier, rather than lattice heating
as previously assumed in the two-temperature models.
