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If a chemical structure will not parse, first determine whether the problem is invalid SMILES syntax, a chemically inconsistent atom or bond assignment, or a difference in how the selected toolkit handles format extensions. Read the full error, inspect the atom or character it identifies, and make one chemistry-supported change at a time. A successful parse confirms that the toolkit accepted the input; it does not prove the structure matches the source you intended.
What a parser failure can mean
For SMILES, failure can occur at different stages. The parser may be unable to read the notation or build the molecular graph—for example, because a branch or ring closure is incomplete. Alternatively, it may build a graph but reject its chemistry during validation, such as when an atom’s specified bonds and hydrogens exceed permitted valence or an aromatic system cannot be assigned a Kekulé form. Some apparent failures are differences in parser settings or extensions rather than errors under every toolkit.
RDKit includes sanitization in its default molecule-reading workflow. Its documentation describes it as “fairly strict” about allowed valences during sanitization. Open Babel offers both a standard parser, which is more forgiving and supports some extensions, and the alternative Smiley parser, which is designed for stricter OpenSMILES compatibility and detailed diagnostics. These tools do not necessarily accept or report every input identically.
Fix the problem systematically
- Save the exact input and complete error. Keep the original string unchanged. Record the full message, including an atom index, character position, warning, or named sanitization step. Change one thing at a time so you can identify which edit addressed the failure.
- Check the notation and graph first. Look for unmatched branch parentheses, ring labels that appear only once, conflicting bond declarations at the ends of a ring closure, and malformed bracket atoms. In documented Smiley examples,
CC(CChas an unmatched opening branch,CC)CCan unmatched closing branch, andC1CCCan unmatched ring bond. A bracket atom can encode isotope, hydrogen count, charge, and stereochemistry; do not remove its brackets or punctuation without checking what those fields mean. - Inspect the flagged atom’s bonds, hydrogens, and charge. For an “explicit valence … greater than permitted” message, check the atom named in the error and add up its specified bond orders and explicit hydrogens. Compare that local environment with the intended structure and formal charge. RDKit’s allowed-valence check uses specified bond orders plus specified hydrogen count.
- Check aromatic heteroatom assignments. Pyridine-like and pyrrole-like nitrogen do not have the same hydrogen assignment, and a toolkit cannot reliably infer the intended placement in every ambiguous case. RDKit’s FAQ gives
c1nccc1as a failure andc1[nH]ccc1as the form for the intended pyrrole-like case. Add[nH]only when the source structure supports that hydrogen. - Confirm the input format and parser settings. Verify that the string is the format expected by the parser, such as SMILES rather than SMARTS, and check for names or annotations after the molecule. RDKit parser parameters can affect whether text after whitespace is treated as a molecule name; disabling CXSMILES and name parsing can instead make extra text an error. Check the parser’s version and supported extensions when two toolkits disagree.
- Parse again, then compare with the source. After each justified correction, reparse and compare connectivity, charge, explicit and implicit hydrogens, aromaticity, and stereochemistry with the source drawing or trusted record. Acceptance by a parser is not an independent identity check.
How to interpret common RDKit errors
“Explicit valence … is greater than permitted”
This usually points to a mismatch between the atom’s specified bond-order sum, explicit hydrogens, formal charge, and the chemistry intended. RDKit’s FAQ uses CN(C)(C)C, which gives neutral nitrogen four single bonds, as an example it rejects. It shows C[N+](C)(C)C for the intended positively charged, four-coordinate nitrogen. That is a correction only if the source structure calls for that charge—not a rule to add a positive charge whenever nitrogen triggers an error.
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RDKit’s Book labels its allowed-valence table “as of 2024.09.1”; the Book itself is the 2026.03.6 documentation. If an element-specific value matters, check the installed RDKit version and the matching documentation rather than assuming the table applies unchanged to every release.
“Can’t kekulize mol”
This means RDKit could not assign a chemically reasonable alternating single- and double-bond form to the submitted aromatic representation under its model. Heteroatom hydrogen and charge assignments can affect that result. Inspect the ring and intended drawing rather than changing aromatic letters or adding hydrogens at random. RDKit’s FAQ explains that, when several heteroatoms could plausibly take a hydrogen, it avoids arbitrarily choosing where the user meant it to go.
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Aromaticity is toolkit-sensitive: conventions and perception rules matter, especially for unusual cases. OpenSMILES requires parsers to check aromatic electron assignment against valence, hydrogens, external bonds, and charges. Lowercase aromatic notation should not be treated as guaranteed to have identical behavior in every toolkit.
Unmatched branch, ring, or bracket syntax
These are notation or graph-construction problems, not evidence that a chemically plausible atom has the wrong valence. Match each opening branch parenthesis with a closing one, ensure ring labels are paired, and inspect bond symbols at both ends of a ring closure. For a bracket atom, check the complete expression—including any isotope, hydrogen count, charge, or stereochemical annotation—before editing it.
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When parsers disagree
Use a parser with detailed diagnostics when the error is vague. Smiley reports specific syntax and semantic problems, including positions; Open Babel describes its standard parser as more forgiving and capable of accepting certain extensions. A string accepted by one and rejected by another is a reason to check the intended format, toolkit version, syntax standard, and extensions. It does not by itself establish which interpretation matches the chemistry.
RDKit can also be asked to read a molecule with sanitization disabled, but this is a diagnostic or advanced workflow, not a normal repair. The resulting object has not undergone chemistry perception, and many RDKit functions may not work properly on it. Partial sanitization also requires care: bypassing a check can defer a real structural problem rather than fix it.
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Documentation for the parser and notation
- Open Babel: Smiley parser documentation — syntax and semantic diagnostics, examples, and the differences between Smiley and the standard parser.
- RDKit FAQ: molecule reading and common errors — examples for explicit valence and kekulization errors; the page shows a March 31, 2023 edit date.
- RDKit Book — parser behavior, sanitization, aromaticity, and valence documentation.
- OpenSMILES specification — rules for SMILES notation, including aromaticity and valence checks.
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