  Ultrafast spectroscopies can access the dynamics of electrons and nuclei at
short timescales, shedding light on nonequilibrium phenomena in materials.
However, development of accurate calculations to interpret these experiments
has lagged behind as widely adopted simulation schemes are limited to
sub-picosecond timescales or employ simplified interactions lacking
quantitative accuracy. Here we show a precise approach to obtain the
time-dependent populations of nonequilibrium electrons and atomic vibrations
(phonons) up to tens of picoseconds, with a femtosecond time resolution.
Combining first-principles electron-phonon and phonon-phonon interactions with
a parallel numerical scheme to time-step the coupled electron and phonon
Boltzmann equations, our method provides unprecedented microscopic insight into
scattering mechanisms in excited materials. Focusing on graphene as a case
study, we demonstrate calculations of ultrafast electron and phonon dynamics,
transient optical absorption, structural snapshots and diffuse X-ray
scattering. Our first-principles approach paves the way for quantitative
atomistic simulations of ultrafast dynamics in materials.
