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Compute the molecular weight (molar mass) of a chemical formula.

Parses formula into an atom tally, looks up each element's mass in the bundled NIST Atomic Weights and Isotopic Compositions database, and returns the total weight in the requested unit. Every call reports all three distinct mass flavors under masses so callers never conflate them or have to re-ask:

• nominal -- sum of the integer mass numbers of the most abundant isotopes,

• average -- the standard atomic weight (average molar mass),

• monoisotopic -- the exact mass of the most abundant isotopes.

The monoisotopic flag selects which of these is mirrored at the top level (weight / uncertainty / formatted) and named by primary. Optionally propagates NIST uncertainties (uncertainty) and/or reports percent composition by mass (composition).

Raises ValueError for an unparseable formula or an unknown element.

Compute the natural isotopic pattern (isotope distribution) of a formula.

Returns the set of isotopologue peaks a mass spectrometer would see: each peak's neutral mass (and mz when charge is non-zero), its intensity relative to the base peak, and its absolute abundance. Also reports the monoisotopic_mass (most-abundant isotope of each element) and the average_mass. Backed by the bundled NIST Atomic Weights and Isotopic Compositions database -- offline and deterministic.

Peaks below threshold percent of the base peak are dropped; at most limit are returned. grouping selects nominal-mass (unit) or fully resolved (exact) peaks.

Raises ValueError for an unparseable formula or an unknown element.

Look up a chemical compound by name or molecular formula.

Searches the bundled name<->formula database (a PubChem subset) first, then the writable user cache, then -- unless disabled -- the online fallback (PubChem, Wikidata, and, when an API key is set, EPA CompTox). A name resolves to its molecular formula(e); a formula resolves to the compound name(s) sharing it (isomers), ordered with the preferred name first. The direction is chosen by by. Returns the query, how it was interpreted (interpreted_as), the normalized Hill-system canonicalization of the query whenever it parses as a formula (e.g. 'O6C6H12' -> 'C6H12O6'; null when it is not a parseable formula), the matches (each {"name", "formula"}, the preferred result first), and the resolving source / license.

Raises ValueError if nothing matches.

Infer a chemical formula from elemental mass percentages (inverse analysis).

The inverse of percent composition: given each element's mass percent -- what a combustion / CHN elemental analyzer reports -- recover the best-fit empirical formula, and, when molar_mass is also known, the molecular formula. Masses come from the bundled NIST standard atomic weights, so the computation is offline and deterministic.

Assumes a 100 g sample, converts each percentage to relative moles via the atomic weights, normalises to whole-number atom ratios (searching integer multipliers up to max_multiplier), and reduces to the empirical formula. Use balance to assign an unmeasured element (typically oxygen) the remainder up to 100%.

Returns the resolved_percentages (including any balance element), the empirical_formula and its weight, the molecular_formula (null without a molar mass) with its molecular_multiplier, and a fit block -- the predicted_percentages the returned formula actually implies, the per-element residuals, and the rms_error / max_abs_error in percentage points. Any data-quality warnings (percentages not summing to 100%, a poor fit, a molar mass far from a whole multiple) are listed.

Raises ValueError on an unknown element, a non-positive percentage, or percentages leaving no remainder for the balance element.