| Structural elements | Bonds | Topological features |
| Carbon | ≤4 | C0, C1, C11, C111, C1111, C2, C12, C112, C22, C3, C13 |
| Oxygen | ≤2 | O0, O1, O11, O2 |
| Nitrogen trivalent | ≤3 | N0, N1, N11, N111, N12, N3 |
| Nitrogen pentavalent | ≤5 | N122 |
| Phosphor pentavalent | ≤5 | P1112 |
| Fluorine | 1 | F1 |
| Chlorine | 1 | Cl1 |
| Bromine | 1 | Br1 |
| Lodine | 1 | L1 |
| Sulfur divalent | ≤2 | S0, S1, S11, S2 |
| Sulfur hexavalent | ≤6 | S111111, S1122 |
| Hydrogen | 1 | H1 = sum (maximal bonds-explicit bonds) |
| ADDITIONAL | ||
| Cl1^C11 | Chlorine linked to C11 | |
| NCO | N1 or N11 or N111 linked to (a carbon linked to O2) | |
| SSSR | Smallest sum of the smallest rings |