Identifying Local Interfaces.

While interfacial regions often occupy a relatively small portion of a system, physical and chemical processes often proceed differently within them. It is therefore useful to identify interfacial regions to answer many questions in physical chemistry. Thermodynamic phases are often described by their density and local structure; therefore, interfacial regions can then be defined as regions with densities and structures that deviate from the properties of the neighboring phases. Using this perspective of local density and structure around an atom, we describe a "directed search cone" method that has proved useful in identifying atoms that sit at the interface between two regions of a system. We call the set of atoms found to be sitting on the surface "leading atoms", and we construct an interface from these atoms that we call the "leading layer interface". We demonstrate the leading layer interface on solid-vacuum, liquid-vacuum, and liquid-vapor systems. In addition to presenting our method and example calculations, we discuss some observations of local density fluctuations that may be useful for the analysis of heterogeneous systems. Depending on the circumstances, there are various perspectives of an interface that may be insightful, and our leading layer interface will be useful in situations where the correlation between interfacial dynamics and local molecular composition is investigated.