Various spectral similarity features can be used. Basic features indicate the presence/absence of expected fragment ions and their m/z deviation, which does not require predicted fragment ion intensities. Additionally, various types of spectral correlation can be used based on predicted fragment ion intensities, such as pearson correlation, spearman correlation, mean square error, cosine similarity , normalized spectral contrast angle (SA) , spectral entropy , and others. Besides calculating such scores for the full spectrum, some PSM rescoring tools also include similar features separately for b-and y-ions or when only considering the top-k peaks.