Zhang Jianguo, Guo Hongxia
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
J Phys Chem B. 2014 May 1;118(17):4647-60. doi: 10.1021/jp411615f. Epub 2014 Apr 22.
In this paper, the transferability of the coarse-grained (CG) force field originally developed for the liquid crystal (LC) molecule 5CB ( Zhang et al. J. Phys. Chem. B 2012 , 116 , 2075 - 2089 ) was investigated by its homologues 6CB and 8CB molecules. Note that, to construct the 5CB CG force field, we combined the structure-based and thermodynamic quantities-based methods and at the same time attempted to use several fragment molecular systems to derive the CG nonbonded interaction parameters. The resultant 5CB CG force field exhibits a good transferability to some extent. For example, not only the experimental densities, the local packing of atom groups, and the antiparallel arrangements of nearest neighboring molecules, but also the unique LC mesophases as well as the nematic-isotropic phase transition temperatures of 6CB and 8CB were reproduced. Meanwhile, the limitations of this 5CB CG force field were also observed, such as the phase transition from nematic to smectic was postponed to the lower temperature and the resulting smectic phase structure is single-layer-like instead of partially interdigitated bilayer-like as observed in underlying atomistic model. Apparently, more attention should be paid when applying a CG force field to the state point which is quite different from which the force field is explicitly parametrized for. The origin of the above limitations can be potentially traced back to the inherent simplifications and some approximations often adopted in the creation process of CG force field, for example, choosing symmetric CG potentials which do not explicitly include electrostatic interactions and are parametrized by reproducing the target properties of the specific nematic 5CB phase at 300 K and 1 atm, as well as using soft nonbonded potential and excluding torsion barriers. Moreover, although by construction this CG force field could inevitably incorporate both thermodynamic and local structural information on the nematic 5CB phase, the anisotropic diffusion coefficient ratios for different LC phases in both 6CB and 8CB systems are reproduced well. All these findings suggest that the multiproperty parametrization route together with fragment-based method provides a new approach to maximize the possibility to simultaneously reproduce multiple physical properties of a given molecule or related molecules with similar chemical structures at other state points.
在本文中，通过其同系物6CB和8CB分子研究了最初为液晶（LC）分子5CB开发的粗粒化（CG）力场的可转移性。请注意，为了构建5CB CG力场，我们结合了基于结构和基于热力学量的方法，同时尝试使用几个片段分子系统来推导CG非键相互作用参数。所得的5CB CG力场在一定程度上表现出良好的可转移性。例如，不仅实验密度、原子团的局部堆积以及最近邻分子的反平行排列，而且6CB和8CB独特的LC中间相以及向列相-各向同性相转变温度都得到了再现。同时，也观察到了这个5CB CG力场的局限性，例如向列相到近晶相的相变被推迟到更低的温度，并且得到的近晶相结构是单层状的，而不是像在底层原子模型中观察到的部分交错双层状。显然，当将CG力场应用于与明确参数化的状态点有很大差异的状态点时，应更加注意。上述局限性的根源可能追溯到CG力场创建过程中经常采用的固有简化和一些近似，例如，选择不明确包含静电相互作用且通过再现300 K和1 atm下特定向列相5CB相的目标性质进行参数化的对称CG势，以及使用软非键势并排除扭转势垒。此外，尽管通过构建这个CG力场不可避免地会纳入向列相5CB相的热力学和局部结构信息，但6CB和8CB系统中不同LC相的各向异性扩散系数比都得到了很好的再现。所有这些发现表明，多性质参数化途径与基于片段的方法一起提供了一种新方法，以最大限度地提高在其他状态点同时再现给定分子或具有相似化学结构的相关分子的多种物理性质的可能性。
