| Resumo: | We have implemented a real-time time-dependent density-functional theory (RT-TDDFT) algorithm within the Siesta method. Building on the basic infrastructure of Siesta we integrate the time-dependent Kohn-Sham equations using the Crank-Nicolson method. Crank-Nicolson integration and other complementary operations are performed in parallel, allowing for the possibility of simulating systems of thousands of atoms. The parallel matrix distribution and manipulation is handled by the ScaLAPACK package, and interfaced to Siesta with the newly-developed MatrixSwitch wrapper package. Parallel scalability tests for our new implementation are performed on a system of 5000 atoms, showing a good scaling up to 316 processes. The direction and impact parameter dependence of electronic stopping power, along with its velocity threshold behavior, is investigated in a prototypical small band gap semiconductor. We calculate the electronic stopping power of H in Ge, a semiconductor with relatively low packing density, using RT-TDDFT. The calculations are carried out in channeling conditions with di erent impact parameters and in di erent crystal directions, for projectile velocities ranging from 0.05 to 0.6 atomic units. The satisfactory comparison with available experiments supports the results and conclusions beyond experimental reach. The calculated electronic stopping power is found to be different in di erent crystal directions; however, strong impact parameter dependence is observed only in one of these directions. The distinct velocity threshold observed in experiments is well reproduced, and its non-trivial relation with the band gap follows a perturbation theory argument surprisingly well. This simple model is also successful in explaining why di erent density functionals give the same threshold even with substantially di erent band gaps. The electronic stopping power of He in Ge is studied within the same framework. Apart from a reasonable agreement with the known experimental results it reproduces the H/He e ect observed in jellium models at low electronic densities. The energy loss to electrons in self-irradiated nickel, a paradigmatic transition metal, is studied. Di erent core states are explicitly included in the simulations to understand their involvement in the dissipation mechanism. The experimental data are well reproduced in the projectile velocity range of 1:0 - 12:0 atomic units. The core electrons of the projectile are found to open additional dissipation channels as the projectile velocity increases. The systematic, explicit, and flexible inclusion of the core states reveals that almost all of the energy loss is accounted for within this first principles approach. Core electrons as deep as 2s are treated explicitly and are found to be necessary to account for the ion energy loss at relatively high projectile velocities. The electronic stopping power of self-irradiated W further con rms the role of core states in accounting for the extremely high electronic stopping values of the transition and heavy metals.
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