Novel method for identifying evaporating, condensing, and reflecting atoms at the argon liquid surface in molecular dynamics simulations

Abstract

We propose a novel approach for identifying evaporating, condensing, and reflecting atoms from the atomistic trajectories obtained from the molecular dynamics simulations. Rather than defining the liquid-vapour interface with the liquid and vapour boundary planes, the liquid phase in this approach is defined as atomic cluster, in which each atom is within the certain cut-off distance from one or more of the atoms of the cluster. 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 reflection events are also registered when the vapour atom enters the liquid phase cluster and then leaves it back to the vapour phase after a short period of time. The liquid-vapour equilibrium molecular dynamics simulation of argon at 85 K temperature showed that the velocity distribution of surface-normal component of evaporation/condensing molecules deviates from the Maxwellian distribution of those molecules that cross the imaginary vapour plane parallel to the liquid surface. The number of reflecting molecules registered with the new approach is considerably lower, which consequently leads to approximately 47% greater value of condensation coefficient compared to the value evaluated with two-boundary interface method. Overall, the proposed method improves over the shortcomings of the commonly used two-boundary interface method, such as the reflections of vapour atoms from the vapour 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 plane film, for example, in aerosol molecular dynamics simulations, where the evaporation/condensation take place simultaneously on many liquid drops/clusters.