Numerous combinations of cations and anions are possible for the production of ionic liquids with fine-tuned properties once the correlation with the molecular structure is known. In this sense, computer simulations are useful tools to explain and even predict the properties of ionic liquids. However, quantum mechanical methods are usually restricted to either small clusters or short time scales so that parameterized force fields are required to study the bulk liquids. In this work, a method is proposed to enable a comparison between the quantum mechanical system and both polarizable and nonpolarizable force fields by means of the calculation of free energy surfaces for the translational motion of the anion around the cation in gas phase. This method was tested for imidazolium-based cations with 3 different anions, [BF4]-, [N(CN)2]-, and [NTf2]-. Better agreement was found with the density functional theory calculations when polarizability is introduced in the force field. In addition, the ion pair free energy surfaces reproduced the main structural patterns observed in the first coordination shell in molecular dynamics simulations of the bulk liquid, proving to be useful probes for the liquid phase structure that can be computed with higher level methods and the comparison with forcefields can indicate further improvements in their parameterization.

中文翻译：

---

离子对自由能表面作为离子液体结构的探针。

一旦知道与分子结构的相关性，就可以将阳离子和阴离子的多种组合用于生产具有微调特性的离子液体。从这个意义上讲，计算机模拟是解释甚至预测离子液体特性的有用工具。但是，量子力学方法通常仅限于小簇或短时间尺度，因此需要参数化的力场来研究散装液体。在这项工作中，提出了一种方法，该方法能够通过计算自由能表面来计算阴离子在气相中围绕阳离子的平移运动，从而在量子力学系统与可极化和不可极化的力场之间进行比较。使用3种不同的阴离子[BF4]-，[N（CN）2]-和[NTf2]-对咪唑鎓阳离子进行了测试。当在力场中引入极化率时，与密度泛函理论计算发现更好的一致性。此外，离子对自由能表面重现了在大分子液体分子动力学模拟中在第一配位壳中观察到的主要结构模式，被证明是液相结构的有用探针，可以用更高水平的方法进行计算和比较。使用力场可以指示其参数设置的进一步改进。