Recently, a new innovative strategy has been developed
and applied to several major database systems. Their
common technique is the implementation of tree-structured
data on substructure searching; they do not use the traditional
fragment-oriented screening method. For example DARC
system,3,4 which has been used online to search the CAS
Registry file (EURECAS) since 1981, uses that sort of data
structure in a screening stage. DARC analyzes structures
in terms of FRELs (Fragment Reduced to an Environment
which is Limited). The FRELs describe two sphere fragments
around a focus that is an atom with at least three nonhydrogen
neighbors. The concept of concentric environments
adopted by FRELs implies that structural data are
stored in a tree form for screening. After the screening
process with tree-formed data, a bit screen match is
conducted mainly to characterize ring systems. A subsequent
atom-by-atom matching eliminates noise data and acquires
hits from the candidates.