NMR Relaxation Rates of Quadrupolar Aqueous Ions from Classical Molecular Dynamics Using Force-Field Specific Sternheimer Factors,Journal of Chemical Theory and Computation

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NMR Relaxation Rates of Quadrupolar Aqueous Ions from Classical Molecular Dynamics Using Force-Field Specific Sternheimer Factors
Journal of Chemical Theory and Computation ( IF 5.7 ) Pub Date : 2021-09-27 , DOI: 10.1021/acs.jctc.1c00690
Iurii Chubak ₁ , Laura Scalfi ₁ , Antoine Carof ₂ , Benjamin Rotenberg ₁

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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 density functional theory (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 nonpolarizable FF with formal ionic charges. Surprisingly, all three FFs considered yield good values for the rates of smaller and less polarizable solutes (Li⁺, Na⁺, K⁺, Cl^–), 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 (Mg²⁺, Ca²⁺, Cs⁺). 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.

中文翻译：

使用力场特定 Sternheimer 因子的经典分子动力学中四极水离子的 NMR 弛豫率

四极原子核的核磁共振 (NMR) 弛豫受其位置处的电场梯度 (EFG) 波动控制。在经典分子动力学 (MD) 中，电子云对 EFG 的贡献可以通过 Sternheimer 近似计算，其中可以使用量子密度泛函理论 (DFT) 计算的原子核处的完整 EFG 被认为与由外部经典电荷分布引起。在这项工作中，我们系统地评估了 Sternheimer 近似的质量以及经典力场 (FF) 对无限稀释时水性四极离子 NMR 弛豫率的影响。特别是，我们比较了使用 ab initio 参数化极化 FF 获得的速率，最近开发的具有缩放离子电荷的经验 FF 和具有正式离子电荷的简单经验非极化 FF。令人惊讶的是，所有三个被考虑的 FF 都对更小和更难极化的溶质（Li⁺ , Na ⁺ , K ⁺ , Cl ^– )，前提是采用了特定于模型的 Sternheimer 参数化。然而，可极化和缩放的电荷 FF 对二价和更可极化的物质（Mg ²⁺、Ca ²⁺、Cs ⁺）。我们发现量子和经典 EFG 之间的线性关系在所有考虑的情况下都很好；然而，这种近似通常会导致结果 EFG 方差中出现相当大的误差，这与计算的速率成正比。我们试图通过对 EFG 包括一阶非线性校正来减少误差，但尚未发现对所得方差的明显改善。后一个结果表明，在离子位置确定 EFG 的更精细方法，特别是那些考虑到离子周围瞬时原子环境的方法，可能对于系统地改进经典 MD 中的 NMR 弛豫率估计是必要的。

更新日期：2021-10-12

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