Many-body screening effects in liquid water

The screening arising from many-body excitations is a crucial quantity for describing ab-sorption and inelastic X-ray scattering (IXS) of materials. Similarly, the electron screening plays a critical role in state-of-the-art approaches for determining the fundamental band gap. However, ab initio studies of the screening in liquid water have remained limited. Here, we use a combined analysis based on the Bethe-Salpeter equation and time-dependent density functional theory. We first show that absorption spectra at near-edge energies are insufficient to assess the accuracy by which the screening is described. Next, when the energy range under scrutiny is extended, we instead find that the IXS spectra are highly sensitive and allow for the selection of the optimal theoretical scheme. This leads to good agreement with experiment over a large range of transferred energies and momenta, and enables establishing the elusive fundamental band gap of liquid water at 9.3 eV.

源于多体激发（many-body excitations）的屏蔽效应是描述材料吸收光谱与非弹性X射线散射（inelastic X-ray scattering，IXS）的关键物理量。此外，电子屏蔽在测定材料基本带隙的前沿理论方法中也发挥着关键作用。然而，针对液态水中屏蔽效应的从头算（ab initio）研究仍相对匮乏。本研究采用基于贝塞耳-萨拉皮特方程（Bethe-Salpeter equation）与含时密度泛函理论的联合分析方法。研究首先表明，仅依托近边能量区域的吸收光谱，无法准确评估屏蔽效应的理论描述精度。随后，当拓展考察的能量范围后，我们发现IXS光谱具有极高的灵敏度，可用于筛选最优的理论计算方案。该方案在宽范围的转移能量与动量区间内均与实验结果高度吻合，并成功确定了长期以来难以精准测定的液态水基本带隙，其值为9.3电子伏特（eV）。