Molecular dynamics simulations at liquid-vapor equilibrium condition and evaporation condition into vacuum were carried out to investigate the boundary condition for the kinetic theory of gases. The determination method for condensation coefficient α consistent with the kinetic theory is also proposed. It was found that α for argon at an equilibrium state is close to unity near the triple point temperature of the bulk liquid, and decreases gradually as the temperature rises. The velocity distribution of molecules evaporating into vacuum becomes nearly half-Maxwellian near the triple point temperature, and is deformed as the temperature rises.

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