Thermodynamic Property Calculation And Composition Analysis Of Liauid Argon

With the continuous improvement of solid theory and gas theory,the theory of liquid,another state of matter,is still an unsolved problem in condensed matter physics.Liquid flows,and in this sense is close to gas.At the same time,interactions between atoms in liquids are as strong as that in solid.The combination of these two features brings about a huge obstacle to the construction of accurate liquid theory.However,the development of the theory of "Mesoscience" based on the energy-minimization multiscale model(EMMS)provides a possible way to solve this problem.The first-order phase transformation of argon system can be analyzed from the perspective of"Mesoscience",that is,there is a regime domination for gas state and solid state,respectively,and the state of liquid can be simply regarded as a mixture of solid-like and gas-like structure,governed by the compromise in competition between these two regimes.Based on this goal,firstly,the molecular dynamics simulation method is used to simulate the argon system,calculate the thermodynamic properties in different states,explore the influence of different cutoff distance,and analyze the underlying physical mechanisms.Then,from the perspective of "Mesoscience",the characteristics of the local inhomogeneous structure of liquid argon system are analyzed,and a novel methodology for the accurate description of simple liquid thermodynamic properties is explored.In Lennard-Jones(LJ)potential argon system investigated by molecular dynamics simulation,more and more calculations suggest to use 4.5? or even larger truncation distances(? is the diameter of argon atom)to obtain the more accurate thermodynamic properties of the systems.Under the condition of NPT ensemble,a larger cutoff distance may more accurately describe the thermodynamic properties and behaviors of the LJ potential argon system.From this perspective,we have studied the influence of different truncation radius on the phase diagram of melting point and boiling point of NPT ensemble of argon system at atmospheric pressure.At the same time,the radial distribution functions(RDF)and velocity autocorrelation function(VACF)at the melting points and different thermodynamic states of the liquid argon with different cutoff distances are analyzed.It is found that,the same thermodynamic properties can be obtained at the corresponding thermodynamic state points with the same proportion of liquid temperature zone under different truncation distances.In the case of constant volume,the cutoff distance has little effect on the results of radial distribution function and velocity autocorrelation function in NVT and NVE ensembles.This work proposes an exploratory way for the selection of the cutoff distance in the simulation of liquid argon,where the truncation distance of 2.5? can meet the requirements of computational accuracy and performance in the simulations.On this basis,the physical mechanisms behind the influence of different truncation radius on the thermodynamic properties are analyzed from the view point of compromise in competition between potential energy and kinetic energy.Furthermore,based on the local number density and diffusion coefficient of the simulation system,the static structure and dynamic heterogeneity of the liquid system are studied.In terms of this inhomogeneous structure,a computational method is proposed to quantitatively describe the proportion of gas-like component and solid-like component in liquid argon with reference to the diffusion coefficient of hard sphere systems under zero external pressure,and the proportion of gas-like component in different-temperature thermodynamic state is calculated according to the phase diagram of NPT argon system under atmospheric pressure.In addition,the solid-liquid-gas phase transition of argon system can be regarded as the result of the compromise in competition between atomic diffusion and vibration.The solid state is dominated by a mechanism A governing atomic vibration,the gas state is dominated by a mechanism B governing atomic diffusion.The liquid can be regarded as a result of the compromise of mechanism A(solid-like)and mechanism B(gas-like)in the competition.This exploration and innovation is different from the traditional thermodynamics.Our research will broaden the application scope of mesoscale science,gain a deeper understanding of the formation mechanism of mesoscale structure,and is also a powerful supplement of the universality of the mesoscience theory.