Inverted bit map

Sheridan et al. ° use similar screens, except that they distinguish atoms based on aromaticity, hybridization, connectivity, and charge at neutral pH. They also calculate screens based on the positions of the lone pairs on heteroatoms and of the center of mass of rings. Thus, they have many more screens. To accommodate this, they use an inverted bit map, that is, one in which each row contains the information on which molecules possess a particular screen. This facilitates searching for example, screening 223,000 structures typically requires about 1 min CPU time on a VAX 8650. However, updating an inverted bit map when a new compound is added is much more demanding of computer resources. 

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