基于经典分子动力学的四极水离子的NMR弛豫速率
NMR relaxation rates of quadrupolar aqueous ions from classical molecular dynamics using force-field specific Sternheimer factors
摘要：
四极核的核磁共振（NMR）弛豫受其位置处的电场梯度（EFG）涨落的控制。在经典的分子动力学（MD）中，电子云对EFG的贡献可以通过Sternheimer近似包括在内，其中可以使用量子DFT计算的核处的完整EFG被认为与外部经典电荷分布所产生的EFG成比例。在这项工作中，我们系统地评估的Sternheimer近似的质量，以及在无限稀释的水四极离子的NMR弛豫速率的经典力场（FF）的影响。特别是，我们比较使用从头算参数化的可极化FF，最近开发的经验FF与缩放的离子电荷和一个简单的经验非可极化FF与正式的离子电荷获得的速率。令人惊讶的是，所有三个FF认为产生良好的值的速率较小和较不极化的溶质，提供了一个模型特定的斯特恩海默参数化。然而，极化和缩放电荷FF产生更好的估计二价和更极化的物种。我们发现，量子和经典的EFG之间的线性关系持有以及在所有的情况下考虑，但是，这样的近似往往会导致相当大的误差，在所得的EFG方差，这是直接成比例的计算速率。我们试图通过包括一阶非线性校正的EFG，以减少错误，但没有明显的改善所产生的方差已经found.The后者的结果表明，更精细的方法，用于确定在离子位置的EFG，特别是那些考虑到周围的离子的瞬时原子环境，可能是必要的，以系统地提高在经典MD的NMR弛豫速率估计。
英文摘要：
The nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is governed by the electric field gradient (EFG) fluctuations at their position. In classical molecular dynamics (MD), the electron cloud contribution to the EFG can be included via the Sternheimer approximation, in which the full EFG at the nucleus that can be computed using quantum DFT is considered to be proportional to that arising from the external, classical charge distribution. In this work, we systematically assess the quality of the Sternheimer approximation as well as the impact of the classical force field (FF) on the NMR relaxation rates of aqueous quadrupolar ions at infinite dilution. In particular, we compare the rates obtained using an ab initio parametrized polarizable FF, a recently developed empirical FF with scaled ionic charges and a simple empirical non-polarizable FF with formal ionic charges. Surprisingly, all three FFs considered yield good values for the rates of smaller and less polarizable solutes, provided that a model-specific Sternheimer parametrization is employed. Yet, the polarizable and scaled charge FFs yield better estimates for divalent and more polarizable species. We find that a linear relationship between the quantum and classical EFGs holds well in all of the cases considered, however, such an approximation often leads to quite large errors in the resulting EFG variance, which is directly proportional to the computed rate. We attempted to reduce the errors by including first order nonlinear corrections to the EFG, yet no clear improvement for the resulting variance has been found. The latter result indicates that more refined methods for determining the EFG at the ion position, in particular those that take into account the instantaneous atomic environment around an ion, might be necessary to systematically improve the NMR relaxation rate estimates in classical MD.
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