体电解质溶液的有限场形式
Finite field formalism for bulk electrolyte solutions
摘要：
电解质溶液对电场的响应方式对于理解这些系统在平衡状态和远离平衡状态时的行为至关重要。这种系统的线性响应理论的目前制定是不一致的，与常见的分子动力学（MD）的实现。使用有限场的形式主义，适合于有限温度MD，我们调查的响应散装NaCl水溶液的有限麦克斯韦（$\mathbf{E}$）和电位移（$\mathbf{D}$）领域。恒定的$\mathbf{E}$哈密顿量使我们能够以与常规MD模拟中使用的力一致的简单方式导出离子电导率的线性响应关系。在恒定$\mathbf{E}$下电解质溶液的简单点电荷模型的模拟在无限稀释下产生在实验值的15%内的电导率。有限场方法还允许我们从其极化响应测量溶剂的介电常数，这被认为是随着离子强度的增加而降低。从极化响应与极化波动测量的介电常数的比较，使直接评估的动态贡献，这个介电衰减，我们发现是小的，但并非微不足道。使用恒定的$\mathbf{D}$制定，我们还重新推导的Stillinger-Lovett条件，其中严格限制溶剂和离子极化波动之间的耦合。
英文摘要：
The manner in which electrolyte solutions respond to electric fields is crucial to understanding the behavior of these systems both at, and away from, equilibrium. The present formulation of linear response theory for such systems is inconsistent with common molecular dynamics (MD) implementations. Using the finite field formalism, suitably adapted for finite temperature MD, we investigate the response of bulk aqueous NaCl solutions to both finite Maxwell ($\mathbf{E}$) and electric displacement ($\mathbf{D}$) fields. The constant $\mathbf{E}$ Hamiltonian allows us to derive the linear response relation for the ionic conductivity in a simple manner that is consistent with the forces used in conventional MD simulations. Simulations of a simple point charge model of an electrolyte solution at constant $\mathbf{E}$ yield conductivities at infinite dilution within 15% of experimental values. The finite field approach also allows us to measure the solvent's dielectric constant from its polarization response, which is seen to decrease with increasing ionic strength. Comparison of the dielectric constant measured from polarization response versus polarization fluctuations enables direct evaluation of the dynamic contribution to this dielectric decrement, which we find to be small but not insignificant. Using the constant $\mathbf{D}$ formulation, we also rederive the Stillinger-Lovett conditions, which place strict constraints on the coupling between solvent and ionic polarization fluctuations.
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