SMILES (Simplified molecular-input line-entry system) allows you to build molecules through a string of text. If you have a SMILES string (e.g., copied from a paper or website) or prefer to enter one for a complicated molecule, Avogadro will build a 3D geometry from the SMILES.
Under the “Build” menu, hold your cursor over “Insert”, and select “SMILES…”.
Enter your SMILES fragment, and select “OK”.
There it is..