An additional concept that has been introduced to facilitate the interpretation of mass spectra, and subsequently structure identification, is that of fragmentation trees . A fragmentation tree is derived computationally from tandem mass spectra using optimization algorithms such that its nodes correspond to fragments or precursor ions and the edges correspond to fragmentation reactions. Fragmentation trees have various uses such as identifying the molecular formula and clustering molecules by aligning fragmentation trees . They have also been used for the prediction of molecular fingerprints that are subsequently used to search structural databases . The information in a mass spectrum is thought to be insufficient to explain the fragmentation process by itself while the fragmantation tree provides complementary information by elucidating the dependencies between the mass peaks . However, fragmentation trees are expensive to compute and often approximations are preferred for practical applications.