The experimental database mentioned in the previous section was the bioconcentration database which contains 1007 compounds that were derived from literature with their corresponding experimental logBCF values. These values were converted to BCF class either as bio-accumulative or non-bio-accumulative compounds with the threshold of logBCF of 3. The source of the database is qsardb.org . A subset of compounds that contained only C, O, N, S, F and Cl atoms was chosen for further research. The original database was reduced from 1007 to 868 compounds, and which composition (as a distribution and elements allocation) is shown in Figure . Among these compounds, 681 were classified as non-bio-accumulative and 187 as bio-accumulative. Since the substructures database is not exhaustive in terms of all possible atoms combinations, it is possible that there might be compounds where no substructures would be found. Such case appeared with 12 compounds, thus the dataset was further reduced to 856 compounds. The compounds that lacked substructures are listed in the supplementary material.