In contrast to the existing two-boundary interface methods, we suggest a new approach for identifying the phase-changing atoms, namely, the evaporating and condensing atoms from the atomistic trajectories obtained from the molecular dynamics simulations. In this approach, the liquid phase is defined as atomic cluster, in which the atoms are within each other by the distance related with the length of structure in the liquid phase. The evaporation and condensation events are registered as the atoms leave or enter the liquid phase cluster, and the atomistic information is collected for each event. The liquid-vapor equilibrium molecular dynamics simulation of argon at 90 K temperature showed that the evaporation events take place in a coordinate range, which take most of the interface defined by the liquid and vapor boundaries. Furthermore, the velocity distribution of surface-normal component of molecules leaving/entering the liquid phase cluster deviates from the Maxwellian distribution of those molecules that cross the imaginary vapor plane placed arbitrary distance from the liquid surface and parallel to the surface.
Overall, the proposed method improves over the shortcomings of the commonly used two-boundary interface method, such as the reflections of vapor atoms from the vapor phase molecules that take place far from liquid phase or the evaluated lower condensation coefficients values due to these types of reflections events. Furthermore, this approach could be used to investigate phase chance processes in simulations, in which the liquid drop is more complex than simple film or sphere, for example, in aerosol molecular dynamics simulations, where the evaporation/condensation take place simultaneously on many liquid drops/clusters.