Time-dependent quantum dynamical simulations of C2 condensation under extreme conditions.

We report theoretical studies of the initial phase of bulk C(2) condensation into carbon nano-structures by means of Born-Oppenheimer and time-dependent quantum mechanical Liouville-von Neumann molecular dynamics based on the density-functional tight-binding (DFTB) framework for electrons. We observe that the time-dependent quantum mechanical approach leads to faster formation of carbon nanostructures than analogous Born-Oppenheimer simulations. Our results suggest that the condensation of bulk carbon is nonadiabatic in nature, with the critical role of electronic stopping as in ion-irradiation of materials. Contrary to time-dependent quantum mechanical simulations, Born-Oppenheimer dynamics incorrectly predict that the short carbon chains obtained from initial reactive collisions between C(2) quickly evaporate, leading to much lower probability of secondary collisions and condensation. We also discuss some deficiencies in Born-Oppenheimer dynamics that lead to unphysical charge polarization and electron transfer.