Efficient ab initio calculation of electronic stopping in disordered systems via geometry pre-sampling: Application to liquid water.
Efficient ab initio calculation of electronic stopping in disordered systems via geometry pre-sampling: Application to liquid water.
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DOI：
10.1063/5.0014276
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发表时间：
2020-06
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通讯作者：
B. Gu;B. Cunningham;Daniel Muñoz Santiburcio;F. Da Pieve;E. Artacho;J. Kohanoff
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作者：
B. Gu;B. Cunningham;Daniel Muñoz Santiburcio;F. Da Pieve;E. Artacho;J. Kohanoff
了解快速离子Se(v)的电子停止曲线，特别是在布拉格峰附近，对于理解辐射损伤是重要的。然而，在实验上，确定光离子的这种特征是非常具有挑战性的，特别是在液态水和生物组织等无序系统中。实时时变密度泛函理论（rt-TDDFT）的最新发展使得沿纳米尺寸轨迹计算Se(v)成为可能。然而，获得一个有意义的统计平均Se(v)，可以与观测值进行比较，仍然是一个挑战。在这项工作中，利用弹丸探测的局部电子结构与弹丸到目标原子的距离之间的相关性，我们设计了一种轨迹预采样方案，从几何上选择一小组短轨迹来加速通过rt-TDDFT计算的平均Se(v)的收敛。对于液态水中的质子，我们首先计算了质子到最近氧原子的距离的参考概率分布函数（PDF），其长度与实验中采样的轨迹相似。然后，依次选择短轨迹，使累积的PDF以越来越高的精度再现ϕR（rp→O）。使用这些预采样轨迹，我们证明了平均Se（vp）在整个速度范围内收敛少于8个轨迹，而其他使用随机和均匀分布轨迹的平均方法需要大约10倍的计算量。这使我们能够将Se（vp）曲线与实验数据进行比较，并根据布拉格规则评估广泛使用的经验表。
Knowledge of the electronic stopping curve for swift ions, Se(v), particularly around the Bragg peak, is important for understanding radiation damage. Experimentally, however, the determination of such a feature for light ions is very challenging, especially in disordered systems such as liquid water and biological tissue. Recent developments in real-time time-dependent density functional theory (rt-TDDFT) have enabled the calculation of Se(v) along nm-sized trajectories. However, it is still a challenge to obtain a meaningful statistically averaged Se(v) that can be compared to observations. In this work, taking advantage of the correlation between the local electronic structure probed by the projectile and the distance from the projectile to the atoms in the target, we devise a trajectory pre-sampling scheme to select, geometrically, a small set of short trajectories to accelerate the convergence of the averaged Se(v) computed via rt-TDDFT. For protons in liquid water, we first calculate the reference probability distribution function (PDF) for the distance from the proton to the closest oxygen atom, ϕR(rp→O), for a trajectory of a length similar to those sampled experimentally. Then, short trajectories are sequentially selected so that the accumulated PDF reproduces ϕR(rp→O) to increasingly high accuracy. Using these pre-sampled trajectories, we demonstrate that the averaged Se(vp) converges in the whole velocity range with less than eight trajectories, while other averaging methods using randomly and uniformly distributed trajectories require approximately ten times the computational effort. This allows us to compare the Se(vp) curve to experimental data and assess widely used empirical tables based on Bragg's rule.