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Cheminformatics is the use of information-handling and computational methods to manage and evaluate information about chemical structures. It covers much more than drawing molecules: it includes storing, indexing, searching, linking, and analyzing molecular records, as well as preparing chemical data for machine learning.
What is cheminformatics?
IUPAC defines cheminformatics as “the science of handling, indexing, archiving, searching, and evaluating information that is specific to chemical structures and is used in data mining, information retrieval, information extraction, and machine learning.” IUPAC Gold Book: cheminformatics
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In practice, that scope can include turning a drawn molecule into a digital structure, searching for matching or related structures, linking records across databases, calculating molecular descriptors, and preparing features for computational analysis. A project may use only some of these methods; cheminformatics is the broader discipline that makes chemical structure data usable.
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RDKit is one example of a cheminformatics toolkit, not a requirement. Its documented capabilities include molecular operations and descriptor generation, with interfaces for multiple programming languages and a PostgreSQL cartridge. For reproducible work, record the toolkit version used; the documentation consulted is labeled 2026.03.6.
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How chemical structures are represented in digital systems
A molecule can be represented in different ways for different jobs. A drawing is convenient for people to inspect. A connection table records atoms and bonds. A line notation such as SMILES serializes structural information as text. An identifier, by contrast, is often used to refer to a chemical record and help link information about it.
SMILES represents a structure as text
SMILES is a line notation for molecular structures. Its exact content depends on the variant and how the structure is encoded. PubChem distinguishes full SMILES, which includes stereochemical and isotopic information, from Connectivity SMILES, which retains connectivity but omits those details. Those variants therefore do not express the same level of structural specificity. PubChem: SMILES
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InChI is an identifier for linking records
IUPAC describes InChI as a non-proprietary identifier that can be used in print and electronic sources to facilitate links among diverse data compilations. It serves a different purpose from a line notation such as SMILES: one is primarily an identifier, while the other is a way to encode a structure. Neither should be treated as a guarantee that all records preserve every detail relevant to a particular chemical question. InChI Trust: Technical FAQ
Choose a representation for the task
- To inspect a molecule: use a drawing or a structure view.
- To exchange structural data as text: use a supported notation such as SMILES or InChI.
- To find a pattern within structures: use a substructure-capable query such as SMARTS, if the database supports it.
- To link records across sources: use an identifier accepted by those sources, while checking what structural details their records and matching rules preserve.
Before comparing or searching records, consider whether stereochemistry, isotopes, charge, and other distinctions matter to the task; which formats the receiving system accepts; and whether the search is exact, similarity-based, or substructure-based. Identical-looking strings or matching identifiers do not automatically prove that records agree in every chemically relevant respect.
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What are molecular descriptors?
A molecular descriptor is a named value associated with a molecular structure, often calculated from that structure. Descriptors can summarize selected features rather than describe a molecule in full. Examples documented by PubChem include molecular formula, molecular mass, exact mass, and rotatable-bond count. Its descriptor data identify a value’s type and, where relevant, its unit. PubChem: RDF data
A descriptor value is not necessarily an experimental measurement. A value calculated from a structure, a value supplied by a database, an experimental result, and a prediction from a separate model have different provenance. When reporting a value, identify which kind it is and cite the calculation method or database record. A descriptor alone does not establish a biological outcome or guarantee how a molecule will behave.
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How are molecular descriptors calculated?
In RDKit, descriptor functions calculate values from a molecule object. The descriptor calculator can provide descriptor names, summaries, and calculator versions. The selected descriptors and the toolkit’s implementation therefore matter when interpreting or comparing results. RDKit: Descriptor Calculation
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Two-dimensional descriptors can be calculated from structural information, but geometry-derived three-dimensional descriptors require coordinates. RDKit’s 3D descriptor module calculates values from a molecular conformer and fails if the molecule has none. A SMILES string by itself does not provide the conformer coordinates needed for such calculations. RDKit: Descriptor Calculation
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What to record for a reproducible calculation
- Describe how input structures were parsed and standardized, including how relevant stereochemistry and isotope information was handled.
- Name the toolkit and version, the descriptors calculated, and their units.
- State whether descriptors are two-dimensional or three-dimensional. For 3D calculations, document how conformers were generated or selected.
- Explain how missing, invalid, or unsupported structures were handled.
- Identify whether each reported value was calculated, supplied by a database, measured experimentally, or predicted by a separate model.
How chemical databases search molecular structures
PubChem illustrates how a chemical database can accept several kinds of query. Its documentation describes searching with names, identifiers, and structure inputs, including SMILES, SMARTS, InChI, molecular formula, and supported chemical structure files. A user can also draw a structure or search from an existing record. The available input and matching behavior depend on the selected search mode. PubChem: Search
Structure-search settings matter. PubChem documents matching thresholds and caveats involving stereochemistry and isotopes, so a result set depends on both the input representation and the chosen matching rules. A search for a connection pattern is not the same question as an exact match, and a similarity search is different again. PubChem: Search
Database record types also matter. PubChem documents compound descriptors separately from substance version descriptors. A compound record and a depositor’s substance record are related concepts, but they are not necessarily the same type of record. PubChem: RDF data
Why representation, descriptors, and provenance belong together
A descriptor is calculated from or associated with a particular representation of a structure, under particular software and settings. A difference in input handling, preserved stereochemistry, descriptor definition, units, toolkit version, or conformer can affect what a value means. When comparing data across databases or software, compare the structures and calculation context—not just the descriptor names or displayed numbers.
There is no universally best molecular representation or descriptor set. The right choice follows from the task: the details the question needs, the query semantics the database supports, and the provenance required to interpret the result.
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