Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854-8054, USA.
J Chem Phys. 2011 Dec 7;135(21):214109. doi: 10.1063/1.3657438.
The gauge cell Monte Carlo method is extended to calculations of the incremental chemical potentials and free energies of linear chain molecules. The method was applied to chains of Lennard-Jones beads with stiff harmonic bonds up to 500 monomers in length. We show that the suggested method quantitatively reproduces the modified Widom particle insertion method of Kumar et al. [S. K. Kumar, I. Szleifer, and A. Z. Panagiotopoulos, Phys. Rev. Lett. 66(22), 2935 (1991)], and is by an order of magnitude more efficient for long chains in terms of the computational time required for the same accuracy of chemical potential calculations. The chain increment ansatz, which suggests that the incremental chemical potential is independent of the chain length, was tested at different temperatures. We confirmed that the ansatz holds only for coils above the θ temperature. Special attention is paid to the effects of the magnitude of adsorption potential and temperature on the behavior of single chains in confinements that are comparable in size with the free chain radius of gyration. At sufficiently low temperatures, the dependence of the incremental chemical potential on the chain length in wetting pores is superficially similar to a capillary condensation isotherm, reflecting monolayer formation following by pore volume filling, as the chain length increases. We find that the incremental gauge cell method is an accurate and efficient technique for calculations of the free energies of chain molecules in bulk systems and nanoconfinements alike. The suggested method may find practical applications, such as modeling polymer partitioning on porous substrates and dynamics of chain translocation into nanopores.
量规单元蒙特卡罗方法被扩展到线性链分子的增量化学势和自由能的计算中。该方法应用于 Lennard-Jones 珠链，链长可达 500 个单体，具有刚性调和键。我们表明，所提出的方法定量再现了 Kumar 等人的改进 Widom 粒子插入法[S. K. Kumar, I. Szleifer, and A. Z. Panagiotopoulos, Phys. Rev. Lett. 66(22), 2935 (1991)]，并且对于长链，在相同的化学势计算精度下，计算时间效率提高了一个数量级。链增量假设表明，增量化学势与链长无关，我们在不同温度下对其进行了测试。我们证实，该假设仅适用于θ温度以上的无规线团。特别关注吸附势和温度对与自由链回转半径相当的尺寸的受限单链行为的影响。在足够低的温度下，在润湿孔中，增量化学势随链长的变化与毛细管冷凝等温线表面相似，反映了随着链长的增加，单层形成后再填充孔体积。我们发现，增量量规单元方法是计算本体系统和纳米受限中链分子自由能的准确有效的技术。所提出的方法可能具有实际应用，例如在多孔基底上模拟聚合物分配和链迁移到纳米孔中的动力学。
