RDKit Cheminformatics Toolkit

Overview

RDKit is a comprehensive cheminformatics library providing Python APIs for molecular analysis and manipulation. This skill provides guidance for reading/writing molecular structures, calculating descriptors, fingerprinting, substructure searching, chemical reactions, 2D/3D coordinate generation, and molecular visualization. Use this skill for drug discovery, computational chemistry, and cheminformatics research tasks.

Tested stable baseline (reviewed 2026-10-01): RDKit 2026.03.6, distributed as rdkit==2026.3.6 on PyPI; this is the current stable GitHub/PyPI release. Official installation docs continue to recommend conda-forge for most users, while cross-platform PyPI wheels are published under the rdkit package name. rdkit-pypi is the old PyPI package name and should only appear when maintaining legacy environments.

Installation and Setup

Use uv when installing into an existing Python environment:

uv pip install "rdkit==2026.3.6"

For reproducible chemistry environments, especially when mixing compiled scientific packages, conda-forge remains the upstream recommendation:

conda create -c conda-forge -n my-rdkit-env rdkit
conda activate my-rdkit-env

Avoid installing both conda rdkit and PyPI rdkit/rdkit-pypi into the same environment unless you are deliberately debugging packaging behavior. Mixed installs can make it unclear which binary extension is being imported.

Core Capabilities

Twelve capability areas, each with worked code, are documented in references/core_capabilities.md:

#	Area	Covers
1	Molecular I/O and creation	SMILES, MOL files and blocks, InChI, SDF and SMILES suppliers, multithreaded reading, writers
2	Sanitization and validation	disabling automatic sanitization, manual and partial sanitization, detecting problems first
3	Analysis and properties	atom and bond iteration, ring information and SSSR, chirality and stereochemistry, fragments
4	Descriptors	MW, LogP, TPSA, H-bond donors/acceptors, rotatable bonds, aromatic rings, bulk calculation, drug-likeness
5	Fingerprints and similarity	topological, Morgan/ECFP via rdFingerprintGenerator, MACCS, atom pair, torsion, Avalon; Tanimoto and other metrics; Butina clustering
6	Substructure searching	SMARTS queries, match retrieval, and a library of common patterns
7	Chemical reactions	reaction SMARTS, applying reactions, reaction fingerprints
8	2D and 3D coordinates	depiction, template alignment, ETKDG embedding, force-field optimization, RMSD, constrained embedding
9	Visualization	single and grid images, substructure highlighting, custom drawer options, Jupyter integration, fingerprint bit environments
10	Molecular modification	explicit hydrogens, Kekulization, aromaticity, substructure replacement, charge neutralization
11	Hashes and standardization	Murcko scaffold and canonical hashes, regioisomer hashes, randomized SMILES for augmentation
12	Pharmacophore and 3D features	feature factories and feature extraction

Worked workflows and the performance, thread-safety, and version-sensitivity notes are in references/workflows_and_best_practices.md.

Prefer portable exchange formats (SMILES, SDF) for shared data; for local caches RDKit's binary molecule representation avoids generic pickle.

Preserve chemical meaning

Canonical isomeric SMILES records one molecular graph under a particular RDKit version. It does not merge tautomers, choose a pH-dependent protonation state, remove salts, or resolve unspecified stereo. Preserve the original structure and source ID before any explicit standardization policy. Similarity 1.0 is not an identity test: fingerprints collide and the bundled search ignores chirality unless --chirality is selected. See the tested identity/standardization example in references/workflows_and_best_practices.md.

The helpers retain one-based source IDs through invalid-record filtering, reject empty molecules, and perform no implicit salt stripping or neutralization. SMILES files have no header; RDKit parses a SMILES/CXSMILES structure followed by an optional name. Text outputs retain enhanced stereo groups as CXSMILES; other CX annotations (such as drawing coordinates) are not an archive format. The search accepts one valid query molecule, not a query batch. MACCS always has 167 positions (166 keys plus unused bit 0); --bits applies to other methods. The substructure helper's historical pains option is only five illustrative motifs, not the published PAINS catalogue. Invalid requested patterns abort. Descriptor cutoffs, QED, and alerts are research heuristics, not evidence of safety, potency, bioavailability, or synthetic feasibility.

Common Pitfalls

1. Forgetting to check for None: Always validate molecules after parsing
2. Sanitization failures: Use DetectChemistryProblems() to debug
3. Hydrogen representation: Most 2D descriptors account for implicit H. Use AddHs() before embedding; it does not choose protonation at a specified pH.
4. 2D vs 3D: Generate appropriate coordinates before visualization or 3D analysis. Check the conformer ID returned by EmbedMolecule before accessing coordinates: -1 means embedding failed. For difficult molecules, enable EmbedParameters.trackFailures and inspect GetFailureCounts(); do not send a failed embedding into force-field optimization.
5. SMARTS matching rules: Remember that unspecified properties match anything
6. Thread safety with MolSuppliers: Don't share supplier objects across threads

Resources

references/

This skill includes detailed API reference documentation:

• api_reference.md - Comprehensive listing of RDKit modules, functions, and classes organized by functionality
• descriptors_reference.md - Selected list of available molecular descriptors with descriptions
• smarts_patterns.md - Common SMARTS patterns for functional groups and structural features

Load these references when needing specific API details, parameter information, or pattern examples.

Only the files listed in references/ and scripts/ are bundled local resources. Names such as rdkit, datamol, scipy, and sklearn refer to installable Python packages, not local files in this skill.

scripts/

Example scripts for common RDKit workflows:

• molecular_properties.py - Calculate comprehensive molecular properties and descriptors
• similarity_search.py - Perform fingerprint-based similarity screening
• substructure_filter.py - Filter molecules by substructure patterns

These scripts can be executed directly or used as templates for custom workflows.

Review evidence

Release, official API/source links, runtime coverage, and explicitly illustrative examples are recorded in references/review.md.