分子动力学模拟中氦分子势函数的应用

Potential functions of helium in molecular dynamics

为了能够在关于氦的分子动力学的模拟中准确地使用其势函数,并确定其量纲1分子间相互作用力,通过对比分析可用于氦的势函数的数学形式和使用条件,指出虽然标准Lennard-Jones(LJ-12-6)势函数的物理意义清晰、形式简化、应用广泛,但对于氦而言,分子间的排斥力不再适宜采用原子中心间距r的-12次方的形式,而更适合采用指数形式的排斥项。在具有指数形式排斥项的势函数中,EXP-6势函数较好地反映了液氦原子间的多体相互作用,其吸引项是van der Waals吸引力的主要组成部分;Bruch-McGee组合半经验势函数多具有分段函数的形式,在模拟过程中转变为分子间作用力的过程和最终的形式较复杂;而Tang-Toennies势函数更适合于描述低密度氦气的性质。通过将势函数的物理参数量纲1化,确定量纲1分子间相互作用力,目的是可以在随后的分子动力学模拟中依使用条件采用。

In order to find appropriate form of potential function and determine the dimensionless equation for intermolecular force of helium-4 atoms in molecular dynamics（MD）study,mathematical expressions and application conditions are analyzed.The standard LJ potential function（LJ-12-6）is widely used in the MD simulation for the gas with simple structure because of its simple form and satisfactory precision.For helium,the repulsive part of the potential expressed in minus twelve power of r is inappropriate and exponential function is used instead.EXP-6 potential is one of the potential functions with exponential repulsive interaction and explains the multi-body interaction between liquid helium atoms,in which the attractive part is the main part of the van der Waals attractive interaction.Another potential function with exponential repulsive interaction is the Bruch-McGee semi-empirical piecewise function,but its final expression and application is complex.Tang-Toennies potential function is more appropriate for describing the helium gas with low density.The dimensionless equations of the intermolecular force for helium-4 atoms are obtained based on the dimensional analysis and can be used in the molecular dynamics simulation.