  We present a new velocity-gauge real-time, time-dependent density functional
tight-binding (VG-rtTDDFTB) implementation in the open-source DFTB+ software
package (https://dftbplus.org) for probing electronic excitations in large,
condensed matter systems. Our VG-rtTDDFTB approach enables real-time electron
dynamics simulations of large, periodic, condensed matter systems containing
thousands of atoms with a favorable computational scaling as a function of
system size. We provide computational details and benchmark calculations to
demonstrate its accuracy and computational parallelizability on a variety of
large material systems. As a representative example, we calculate laser-induced
electron dynamics in a 512-atom amorphous silicon supercell to highlight the
large periodic systems that can be examined with our implementation. Taken
together, our VG-rtTDDFTB approach enables new electron dynamics simulations of
complex systems that require large periodic supercells, such as crystal
defects, complex surfaces, nanowires, and amorphous materials.
