Molecular formulas

A Rust crate for parsing, manipulating, and analyzing molecular formulas.

It validates correctly against 120M compounds from PubChem (99.46% mass accuracy) and is fuzzed for over 10 billion iterations (see the fuzz crate) to ensure we handle all sorts of textual input.

Features

molecular-formulas supports nested groups, hydrates, salts, isotope notation, flexible charge notation, and strict InChI-style formula validation. It provides typed formula variants for general chemical formulas, Hill-sorted InChI formula layers, residual groups, and mineral polymorph prefixes. The crate can inspect elements, isotopes, mixtures, Hill ordering, charge, isotopologue mass, molar mass, and m/z values. Element and isotope data come from elements-rs; optional features include serde, arbitrary/fuzzing, mem_size, and mem_dbg. The crate is no_std with alloc unless fuzzing or mem_dbg is enabled.

Installation

Add this to your Cargo.toml:

[dependencies]
molecular-formulas = "0.1.9"

Usage

Here are some examples of how to use the library:

Basic Parsing and Properties

use std::str::FromStr;
use molecular_formulas::prelude::*;

// efficient u16 counters and i16 charge
// Note: You can use u32 or u64 for larger molecules.
let formula: ChemicalFormula = ChemicalFormula::from_str("C6H12O6").unwrap();

println!("Formula: {}", formula);
println!("Monoisotopic Mass: {} Da", formula.isotopologue_mass());
println!("Average Mass: {} Da", formula.molar_mass());
println!("Charge: {}", formula.charge());

Complex Formulas, Hydrates and Ions

The parser handles parentheses, brackets, hydrates (dots), and charges with ease.

use std::str::FromStr;
use molecular_formulas::prelude::*;

// Copper(II) sulfate pentahydrate
let hydrate: ChemicalFormula = ChemicalFormula::from_str("CuSO4.5H2O").unwrap();
assert_eq!(hydrate.to_string(), "CuSO₄.5H₂O");

// An ion with unicode charge notation
let ion: ChemicalFormula = ChemicalFormula::from_str("SO₄²⁻").unwrap();
assert_eq!(ion.charge(), -2.0);

// Recursively nested groups
let complex: ChemicalFormula = ChemicalFormula::from_str("[Co(NH3)5Cl]Cl2").unwrap();

Isotopes

You can specify isotopes using standard notation (superscripts or square brackets).

use std::str::FromStr;
use molecular_formulas::prelude::*;

// Carbon-13 labeled methane
let labeled: ChemicalFormula = ChemicalFormula::from_str("[13C]H4").unwrap();
// or
let labeled_unicode: ChemicalFormula = ChemicalFormula::from_str("¹³CH₄").unwrap();

assert_eq!(labeled, labeled_unicode);

// Check if it contains specific isotopes
let c13 = Isotope::try_from((Element::C, 13_u16)).unwrap();
assert!(labeled.contains_isotope(c13));

OCR-Resistant Parsing

The parser is designed to be robust against common OCR errors and unicode variations, handling multiple types of hyphens, dashes, and dots seamlessly.

use std::str::FromStr;
use molecular_formulas::prelude::*;

// Standard notation
let f1: ChemicalFormula = ChemicalFormula::from_str("CuSO4.5H2O").unwrap();
// OCR error: '｡' (Halfwidth Ideographic Full Stop) instead of '.'
let f2: ChemicalFormula = ChemicalFormula::from_str("CuSO4｡5H2O").unwrap();
assert_eq!(f1, f2);

// Standard charge
let c1: ChemicalFormula = ChemicalFormula::from_str("SO4-2").unwrap();
// OCR error: Using En Dash '–' instead of Minus '-'
let c2: ChemicalFormula = ChemicalFormula::from_str("SO4–2").unwrap();
assert_eq!(c1, c2);

InChI Formula Validation

The library supports strictly validated InChI-style formulas, which enforce Hill notation sorting (C first, H second, then alphabetical).

use std::str::FromStr;
use molecular_formulas::errors::ParserError;
use molecular_formulas::prelude::*;

// Valid Hill-sorted formula
let valid: InChIFormula = InChIFormula::from_str("C2H5O").unwrap();

// Invalid: Not Hill-sorted (O comes before H)
let invalid: Result<InChIFormula, _> = InChIFormula::from_str("C2OH5");
assert_eq!(invalid.unwrap_err(), ParserError::NotHillOrdered);

Validation against PubChem

This library is tested against the PubChem database, which contains over 123 million compounds. This ensures correctness when parsing real-world chemical data.

We validate both ChemicalFormula (mass analysis) and InChIFormula (the formula layer of InChI).

Specifically, we download the CID-Mass.gz and CID-InChI-Key.gz documents, which can be found in the Extras FTP directory of PubChem.

You can run the validation suites yourself:

# Validate Mass Calculation (ChemicalFormula), takes about 55 seconds,
# most of which is just I/O time
cargo test --release --test test_pubchem_validation -- --ignored --nocapture

# Validate InChI Parsing (InChIFormula), takes about 45 seconds,
# most of which is just I/O time
cargo test --release --test test_pubchem_inchi_validation -- --ignored --nocapture

Validation Results (January 2026)

Note: The remaining ~0.5% mismatches are largely attributed to inconsistencies or errors in the source PubChem records rather than parsing errors.

You can find a report of the worst mismatches in worst_mismatches.md.

Benchmarks

This crate includes benchmarks to measure parsing performance for both InChIFormula and ChemicalFormula.

To run the benchmarks:

cargo bench

Current benchmarks cover:

• InChIFormula: Parsing a large mixture string with 76 components (~3.75 µs).
• ChemicalFormula: Parsing a complex formula with unicode subscripts, charges, and multiple elements (C₃₉₀H₄₀₄B₂Br₂ClCs₂F₁₁K₂MnN₂₆Na₂O₁₀₀OsPdS₃W₂³⁻) (~801 ns).

Current Limitations

At this time, the parser does not support and might support in the future:

• Fractional counts (e.g., C1.5H3).

License

This project is licensed under the MIT License. See the LICENSE file for details.