Rivera José L, Douglas Jack F
Laboratorio de Modelamiento y Simulación Molecular, Universidad Michoacana de San Nicolás de Hidalgo Morelia Michoacán 58000 Mexico
Materials Science and Engineering Division, National Institute of Standards and Technology Gaithersburg Maryland 20899 USA.
RSC Adv. 2019 Nov 4;9(61):35803-35812. doi: 10.1039/c9ra07058c. eCollection 2019 Oct 31.
Recent simulation studies of the surface tension , and other properties of thin free-standing films, have revealed unexpected finite size effects in which the variance of the properties vary monotonically with the in-plane width of the films, complicating the extrapolation of estimates of film properties to the thermodynamic limit. We carried out molecular dynamics simulations to determine the origin of this phenomenon, and to address the practical problem of developing a more reliable methodology for estimating in the thermodynamic limit. We find that there are two distinct finite size effects that must be addressed in a finite size analysis of in thin films. The first finite size scale is the in-plane width of the films and the second scale is the simulation cell size in the transverse direction. Increasing the first scale enhances fluctuations in , measured by the standard deviation of their distribution, while increasing the second reduces fluctuations due to a corresponding increased 'freedom' of the film to fluctuate out of plane. We find that using progressively large simulation cells in the transverse direction, while keeping the film width fixed to an extent in which the full bulk liquid zone is developed, allows us to obtain a smooth extrapolation to the thermodynamic limit, enabling a reduction of the uncertainty to a magnitude on the order of 1% for systems having a reasonably large size, , (1 μm).
最近对薄的自支撑膜的表面张力及其他性质的模拟研究揭示了意想不到的有限尺寸效应，其中这些性质的方差随膜的面内宽度单调变化，这使得将膜性质的估计值外推到热力学极限变得复杂。我们进行了分子动力学模拟，以确定这种现象的起源，并解决在热力学极限下开发更可靠的估计方法这一实际问题。我们发现，在薄膜的有限尺寸分析中必须考虑两种不同的有限尺寸效应。第一个有限尺寸尺度是膜的面内宽度，第二个尺度是横向的模拟单元尺寸。增加第一个尺度会增强由其分布的标准差测量的波动，而增加第二个尺度会减少波动，这是由于膜在面外波动的“自由度”相应增加。我们发现，在横向使用逐渐增大的模拟单元，同时将膜宽度固定在一定程度以形成完整的本体液体区域，能使我们顺利外推到热力学极限，对于尺寸合理较大（ ， （1μm））的系统，可将波动的不确定性降低到1%左右。
