Abstract
We present a novel definition of the local and instantaneous interface of the liquid-vapor equilibrium system from the microscopic point of view. In this definition, the density distribution, which is given by the position of discrete particles from molecular dynamics (MD) simulation, is described as the field quantity. In other words, we make the density distribution, which is inherently described by the summation of the delta functions, smoothed by distributing the smoothed delta function on the position of each molecule. The surface position is defined as the position on which the field has the certain density value, and the distance function from the surface can be evaluated by the reinitialization procedure of the level set method. In order to investigate the physical meaning of our proposed definition of the instantaneous interface, we calculated the averaged position of the instantaneous interface, and found that it shows a good agreement with the position of the equimolar surface which is defined thermodynamically in the Gibbs's manner.
