Density functional theory (DFT) calculations were performed on a model of a ferrihydrite nanoparticle interacting with chromate ([Formula: see text]) in water. Two configurations each of monodentate and bidentate adsorbed chromate as well as an outer-sphere and a dissolved bichromate ([Formula: see text]) were simulated. In addition to the 3-D periodic planewave DFT models, molecular clusters were extracted from the energy-minimized structures. Calculated interatomic distances from the periodic and cluster models compare favorably with Extended X-ray Absorption Fine Structure spectroscopy values, with larger discrepancies seen for the clusters due to over-relaxation of the model substrate. Relative potential energies were derived from the periodic models and Gibbs free energies from the cluster models. A key result is that the bidentate binuclear configuration is the lowest in potential energy in the periodic models followed by the outer-sphere complex. This result is consistent with observations of the predominance of bidentate chromate adsorption on ferrihydrite under conditions of high surface coverage (Johnston Environ Sci Technol 46:5851-5858, 2012). Cluster models were also used to perform frequency analyses for comparison with observed ATR FTIR spectra. Calculated frequencies on monodentate, bidentate binuclear, and outer-sphere complexes each have infrared (IR)-active modes consistent with experiment. Inconsistencies between the thermodynamic predictions and the IR-frequency analysis suggest that the 3-D periodic models are not capturing key components of the system that influence the adsorption equilibria under varying conditions of pH, ionic strength and electrolyte composition. Model equilibration via molecular dynamics (MD) simulations is necessary to escape metastable states created during DFT energy minimizations based on the initial classical force field MD-derived starting configurations.

中文翻译：

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铬酸盐吸附到三水铁矿纳米粒子上的密度泛函理论建模。

密度泛函理论（DFT）的计算是在水中与铬酸盐相互作用的水铁矿纳米颗粒模型上进行的。模拟了两种配置，分别是单齿和双齿吸附铬酸盐，外球和溶解的重铬酸盐（[分子式：见正文]）。除了3-D周期平面波DFT模型外，还从能量最小化的结构中提取了分子簇。从周期模型和簇模型计算出的原子间距离与扩展X射线吸收精细结构光谱值相比具有优势，由于模型基底的过度松弛，簇的差异更大。相对势能从周期模型中得出，吉布斯自由能从聚类模型中得出。一个关键的结果是，在周期性模型中，双齿双核构型的势能最低，其次是外层复合物。该结果与在高表面覆盖条件下三价铬酸盐吸附在三水铁矿上占优势的观察结果一致（Johnston Environ Sci Technol 46：5851-5858，2012）。群集模型还用于执行频率分析，以便与观察到的ATR FTIR光谱进行比较。在单齿，双齿双核和外球复合体上计算的频率均具有与实验一致的红外（IR）活性模式。热力学预测与红外频率分析之间的不一致表明，3-D周期模型未捕获在变化的pH条件下影响吸附平衡的系统关键成分，离子强度和电解质成分。必须通过分子动力学（MD）模拟进行模型平衡，以逃避基于初始经典力场MD衍生的初始配置在DFT能量最小化期间​​创建的亚稳态。