SMILES arbitrary target specification

SMILES (Simplified Molecular Input Line Entry System) is a notation system used to represent chemical structures in a way that can be easily read by both humans and computers. The notation consists of a line of text that encodes the atoms, bonds, connectivity, and sometimes stereochemistry of a molecule. **Arbitrary Target Specification (ATS)** refers to a feature in SMILES that allows for the representation of complex chemical structures, including various modifications and functional groups.