基于分子动力学模拟流体输运性质的稳定性分析

利用线性响应理论对Ar流体输运参数进行了分子动力学模拟,结果发现:导热系数和黏度会随着自相关积分函数积分时间的增加而产生剧烈波动,而扩散系数却相对稳定. 针对积分稳定性这一问题,对导热系数和黏度中的热流密度和应力张量进行了分解分析,发现含分子间作用力项是影响稳定性的最大因素. 从牛顿力学出发对作用力项的影响机理进行了分析,指明减小这种影响的最主要方法是使在体系进行统计输运参数前达到稳定平衡状态,即最小的预平衡步数应该满足使体系达到该状态下熵最大或者能量最低,并尽量减小温度对体系的影响. 同时,还对模拟盒尺寸、统计步长等因素对积分稳定性的影响进行了分析,给出了保持稳定性的建议. 关键词： 分子动力学 输运性质 自相关函数 稳定性

The analysis of transport properties stability in molecular dynamics simulations

The Green-Kubo time correlation function is used to predict fluid argon transport properties, such as diffusion coefficient, viscosity and thermal conductivity, through molecular dynamics simulations. The results show that the transport characteristics, especially the viscosity and thermal conductivity, fluctuate intensely during the simulations. The collective stress tensor is separated into two parts, one is due to the kinetic energy and the other is due to the pair virial function, and the collective heat flux vector is contributed from the kinetic energy, the intermolecular potential and the pair virial function. The results show that the transport characteristics, especially the viscosity and the thermal conductivity, fluctuate intensely during the simulations. The most important contribution to the viscosity and the thermal conductivity is from the autocorrelation of the virial term. The calculations indicate that a more compatible integration time step method is needed to reduce instabilities when the Green-Kubo time correlation is used to calculate the fluid transport parameters. Other factors which influence the stability are also discussed in the paper.