Nucleation is the process of forming new condensed-phase particles from a continuous phase, such as gas and vapor. The Particle Tracking feature only considers particles generated via homogeneous nucleation (or self-nucleation); that is, no foreign nucleus is involved in the nucleation process. This constraint is important since we assume that the particle core is made up entirely of one chemical compound, that is, the bulk species. Note that creation of particles by multiple nucleation reations is allowed.