A group contribution model (GCM) was used to predict the YSIs of the terpenes and hydrogenated terpenes. In this model, the sooting tendency of a molecule is calculated as the linear sum of the YSI contributions from each of its component carbon-centered groups. More details about the prediction and the training set are included in SI L. Fig. plots the predicted YSIs against the measured YSIs. The predictions agree reasonably well with the measurements except for three striking outliers: CVC, b-OM, and a-PN. By its nature, the GCM cannot account for non-nearest neighbor interactions (NNNIs); therefore, the observed trend suggests that NNNIs are important for the three outliers, but not the other compounds. Each of these outliers and several related compounds are discussed next.