Representing drugs with chemical functional groups composition. The number of drugs is extremely large. However, most of them are small organic molecules and are composed of some fixed small structures, called functional groups. Since functional groups usually represent the characteristics of a compound as well as its reaction mechanism with other molecules, features derived from its functional groups could be very effective in characterizing a drug. Moreover, the number of common functional groups is quite small, and hence it is possible to use the functional group composition to uniquely represent a drug [48] . A number of functional groups are available in nature, and we selected the following 28 common groups for the current study: (1) alcohol, (2) aldehyde, (3) amide, (4) amine, (5) hydroxamic acid, (6) phosphorus, (7) carboxylate, (8) methyl, (9) ester, (10) ether, (11) imine, (12) ketone, (13) nitro, (14) halogen, (15) thiol, (16) sulfonic acid, (17) sulfone, (18) sulfonamide, (19) sulfoxide, (20) sulfide, (21) a_5c_ring, (22) ar_6c_ring, (23) non_ar_5c_ring, (24) non_ar_6c_ring, (25) hetero ar_6_ring, (26) hetero non_ar_5_ring, (27) hetero non_ar_6_ring, and (28) hetero ar_5_ring. Thus, following the same treatment as in [23] , a drug compound can now be formulated as a 28-D (dimensional) vector given below: