临界点附近CO_(2)物性畸变特性的分子动力学研究

Study on Molecular Dynamics of Singular Nature of Physical Properties near Critical Point in Carbon Dioxide System

通过分子动力学模拟方法,从微观角度研究了临界点附近CO_(2)物性畸变特性。分子动力学模拟表明COMPASS力场在远离临界点时具有较高精度,在临界点附近精度较低但可反映密度畸变现象。通过划分模拟空间,获得体系密度涨落特性,分析表明,临界点附近存在很大的密度涨落,且临界点前后密度涨落值呈现不对称特性;在CO_(2)分子上建立局部坐标,并定义了二聚体构型三参数描述方法,该描述方法能够全面描述二聚体构型分布,模拟结果显示在平行构型、T构型及十字构型中,T构型出现概率最高,3种构型的转变与物性畸变或存在紧密关联。

Based on the molecular dynamics simulations,this paper analyzed the singular nature of physical properties of the carbon dioxide near the critical point.Detailed analysis of density simulation exhibited that an excellent agreement can be obtained by COMPASS forcefield besides the pseudo-critical region,while the peculiar behavior of density can again be observed.The computational domain is divided into the small sub-domains in which the standard deviation are evaluated as density fluctuation,and it shows maximum near the critical point and higher values at supercritical condition than that at subcritical condition.A new 3-parameter model was put forward to define the CO_(2) dimer distribution after each molecule in the domain had been supplied with the local coordination frame.The result shows that the T-shaped dimer is with higher probability to be formed than the parallel and crossed configuration,and the conversion of these dimer conformations closely associates with the peculiar properties of the critical point.