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 subpicosecond 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 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.

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进行精确模拟电子和材料中原子振动的超快动力学

超快光谱学可以在短时间范围内访问电子和原子核的动力学，从而避免了材料中的非平衡现象。但是，用于解释这些实验的精确计算的开发已经落后了，因为广泛采用的模拟方案仅限于皮秒级以下的时间范围，或者采用缺乏定量精度的简化交互作用。在这里，我们展示了一种精确的方法，可获取高达数十皮秒的非平衡电子和原子振动（声子）随时间变化的总体，且具有飞秒的时间分辨率。将第一性原理的电子-声子和声子-声子相互作用与并行数值方案结合起来，以时间步长耦合电子和声子的玻尔兹曼方程，我们的方法为激发材料的散射机理提供了微观见解。以石墨烯为案例研究，我们演示了超快电子和声子动力学，瞬态光吸收，结构快照和漫射X射线散射的计算。我们的第一原理方法为材料超快动力学的定量原子模拟铺平了道路。