Feature Definitions and Pharmacophore Representation

General definitions may result in models that are universal, at the cost of restrictivity. However, restrictivity is an important issue in pharmacophore searching and, therefore, feature descriptions that are too general need to be 

4 Chemical functionality without geometric constraint, e.g., an H-bond acceptor without a projected point or a lipophilic group + + + - 

3 Chemical functionality (H-bond acceptor, H-bond donor, positive ionizable, negative ionizable, hydrophobic) with geometric constraint, e.g., an H-bond acceptor vector including an acceptor point as well as a projected donor point aromatic ring including a ring plane + + + 

1 Molecular graph descriptor (atom, bond) with geometric constraint, e.g., a phenol group facing a parallel benzenoid system within a distance of 2-4 A + + + 

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