Carbone Paola, Varzaneh Hossein Ali Karimi, Chen Xiaoyu, Müller-Plathe Florian
Eduard-Zintl-Institut für Anorganishe und Physikalische Chemie, Technische Universität Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany.
J Chem Phys. 2008 Feb 14;128(6):064904. doi: 10.1063/1.2829409.
A key question for all coarse-graining methodologies is the degree of transferability of the resulting force field between various systems and thermodynamic conditions. Here we present a detailed study of the transferability over different thermodynamic states of a coarse-grained (CG) force field developed using the iterative Boltzmann inversion method. The force field is optimized against distribution functions obtained from atomistic simulations. We analyze the polymer case by investigating the bulk of polystyrene and polyamide-6,6 whose coarse-grained models differ in the chain length and in the number of atoms lumped in one bead. The effect of temperature and pressure on static, dynamic, and thermodynamic properties is tested by comparing systematically the coarse-grain results with the atomistic ones. We find that the CG model describing the polystyrene is transferable only in a narrow range of temperature and it fails in describing the change of the bulk density when temperature is 80 K lower than the optimization one. Moreover the calculation of the self-diffusion coefficient shows that the CG model is characterized by a faster dynamics than the atomistic one and that it overestimates the isothermal compressibility. On the contrary, the polyamide-6,6 CG model turns out to be fully transferable between different thermodynamic conditions. The transferability is checked by changing either the temperature or the pressure of the simulation. We find that, in this case, the CG model is able to follow all the intra- and interstructural rearrangements caused by the temperature changes. In addition, while at low temperature the difference between the CG and atomistic dynamics is remarkable due to the presence of hydrogen bonds in the atomistic systems, for high temperatures, the speedup of the CG dynamics is strongly reduced, leading to a CG diffusion coefficient only six times bigger than the atomistic one. Moreover, the isothermal compressibility calculated at different temperatures agrees very well with the experimental one. We find that the polymer chain length does not affect the transferability of the force field and we attribute such transferability mainly to the finer model used in describing the polyamide-6,6 than the polystyrene.
对于所有粗粒化方法而言，一个关键问题是所得力场在不同系统和热力学条件之间的可转移性程度。在此，我们对使用迭代玻尔兹曼反演方法开发的粗粒化（CG）力场在不同热力学状态下的可转移性进行了详细研究。该力场是根据从原子模拟获得的分布函数进行优化的。我们通过研究聚苯乙烯和聚酰胺 - 6,6的本体来分析聚合物情况，它们的粗粒化模型在链长以及每个珠子中聚集的原子数方面有所不同。通过系统地将粗粒化结果与原子模拟结果进行比较，测试了温度和压力对静态、动态和热力学性质的影响。我们发现，描述聚苯乙烯的CG模型仅在狭窄的温度范围内可转移，并且当温度比优化温度低80 K时，它无法描述本体密度的变化。此外，自扩散系数的计算表明，CG模型的动力学比原子模型更快，并且它高估了等温压缩率。相反，聚酰胺 - 6,6的CG模型在不同热力学条件之间完全可转移。通过改变模拟的温度或压力来检查可转移性。我们发现，在这种情况下，CG模型能够跟踪由温度变化引起的所有内部和结构间的重排。此外，虽然在低温下，由于原子系统中存在氢键，CG动力学与原子动力学之间的差异显著，但在高温下，CG动力学的加速作用大大降低，导致CG扩散系数仅比原子扩散系数大六倍。此外，在不同温度下计算的等温压缩率与实验值非常吻合。我们发现聚合物链长不会影响力场的可转移性，并且我们将这种可转移性主要归因于用于描述聚酰胺 - 6,6的模型比聚苯乙烯的模型更精细。
