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

We report theoretical studies of the initial phase of bulk C₂ 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 C₂ 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.