mol2lewis

Convert various chemical identifiers (SMILES, formulas, CIDs, names, InChI, InChIKey) to ChemFig LaTeX code with Lewis structures, including lone pairs and draft mode annotations. It's based on mol2chemfig package and the python3 version. It only works for relatively easy molecules. It can't be working with molecules with hundreads of elements.

Features

• Universal Input: Automatically recognizes SMILES, PubChem CIDs, InChI, InChIKey, IUPAC names, and file paths
• Automatic Lone Pairs: Adds proper lone pair placement based on 2D molecular geometry
• Draft Mode: Visual markers for bonds and lone pairs during development
• Powered by mol2chemfig: Uses the mol2chemfig command-line tool for structure generation

Installation

Prerequisites

• Python 3.8+
• RDKit
• mol2chemfig command-line tool (installed via mol2chemfigPy3 package)

Install

pip install mol2lewis

This will automatically install RDKit, mol2chemfigPy3 (which includes the mol2chemfig command), and pubchempy.

Usage

The lewis command returns a list of python dictionaries. One dictionary for each isomer. So, if only one isomer is available, only one dictionary will appear in the list.

Each dictionary has four keys: 'normal', 'draft', 'iupac_name', and 'name', corresponding to the ChemFig code in normal/draft modes, the IUPAC name, and the common name (from PubChem).

Draft mode adds red visual markers to bonds for educational purposes.

Simple Examples

from mol2lewis import lewis

# From SMILES
code = lewis("CCO")  # Ethanol
print(code[0]['normal'])    # chemfig code for the lewis code

will give you

\chemfig{
H
-[:343.9]C
(
    -[:73.9]H
)
(
    -[:253.9]H
)
-[:343.9]C
(
    -[:253.9]H
)
(
    -[:73.9]H
)
-[:343.9]\charge{224=\|,344=\|}{O}
-[:43.9]H
}

and

print(code[0]['draft'])    # chemfig code for the lewis code

will give you

\chemfig{
\charge{344=\red}{H}
-[:343.9]\charge{344=\red,164=\red,74=\red,254=\red}{C}
(
    -[:73.9]\charge{254=\red}{H}
)
(
    -[:253.9]\charge{74=\red}{H}
)
-[:343.9]\charge{164=\red,344=\red,254=\red,74=\red}{C}
(
    -[:253.9]\charge{74=\red}{H}
)
(
    -[:73.9]\charge{254=\red}{H}
)
-[:343.9]\charge{164=\red,44=\red,224=\:,344=\:}{O}
-[:43.9]\charge{224=\red}{H}
}

isomers = lewis("C2H6O", selection='all')  # All isomers of C2H6O
print(isomers[0]['iupac_name'])  # IUPAC name: ethanol
print(isomers[0]['name'])       # Common name: ethanol
print(isomers[1]['iupac_name'])  # IUPAC name: dimethyl ether
print(isomers[1]['name'])       # Common name: methoxymethane

The same code can be obtained from other input types:

# From PubChem CID (numeric)
code = lewis(702)  # Ethanol

# From InChI
code = lewis("InChI=1S/C2H6O/c1-2-3/h3H,2H2,1H3")  # Ethanol

# From InChIKey
code = lewis("LFQSCWFLJHTTHZ-UHFFFAOYSA-N")  # Ethanol

# From IUPAC name (via PubChem lookup)
code = lewis("ethanol")

# From file
code = lewis("molecule.mol")
code = lewis("structure.smi")

# From molecular formula (various isomers can be returned)
code = lewis("C2H6O")  # First isomer
codes = lewis("C2H6O", selection='all')  # All isomers
codes = lewis("C2H6O", selection='first_n', n=3)  # First 3 isomers

LaTeX Requirements:

If you objective is only to get the pdf of the molecule, you can just compile the LaTeX code below :

\documentclass{standalone}
\usepackage{chemfig}
\usepackage{mol2lewis}  % Required only for draft mode
\begin{document}

...the chemfig molecule code here...

\end{document}

The mol2lewis.sty LaTeX package is automatically included in the Python package installation for convenience.

LaTeX Package Installation:

After installing mol2lewis via pip, you can retrieve the mol2lewis.sty file from the Python installation:

# Find where pip installed mol2lewis
pip show mol2lewis

# Copy the .sty file to your LaTeX directory
cp $(python -c "import mol2lewis; import os; print(os.path.dirname(mol2lewis.__file__))")/mol2lewis.sty /your/latex/directory/

Alternatively, simply download the file from the GitHub repository and place it in your LaTeX directory.

With Options

# Rotate molecule
rotated = lewis("CCO", angle=45)

# Aromatic circles (default: False)
aromatic = lewis("c1ccccc1", aromatic_circles=True)

# Hide carbon atoms (default: False)
no_carbons = lewis("CCO", show_carbons=False)

# Combine options
code = lewis(702, angle=30, aromatic_circles=True, draft=True)

Python API Reference

Main Function

lewis(input, **options)

Universal converter that automatically recognizes input type (SMILES, CID, InChI, InChIKey, name, or file path).

Parameters:

• input (str or int): Chemical identifier
• input_type (str): auto (default), smiles, formula or name. Forces the interpretation of ambiguous strings: CO is methanol as a SMILES but carbon monoxide as a formula (input_type='formula')
• angle (float): Rotation angle (default: 0.0)
• relative_angles (bool): Use relative bond angles from mol2chemfig (default: False)
• aromatic_circles (bool): Draw aromatic circles (default: False)
• show_carbons (bool): Show carbon symbols (default: True)
• show_methyls (bool): Show methyl groups (default: False)
• flip (bool): Horizontal flip (default: False)
• flop (bool): Vertical flip (default: False)
• selection (str): For formulas, one of first, random, all, first_n (default: first)
• n (int): Number of molecules when selection='first_n'
• enumerate_stereo (bool): Enumerate stereoisomers locally with RDKit (default: False)
• wrap_chemfig (bool): Wrap output in \\chemfig{...} (default: True)

Returns: A list of dictionaries with 'normal', 'draft', 'iupac_name', and 'name' keys. Each dictionary corresponds to an isomer. Returns an empty list on failure. A recognised CID, InChI, InChIKey or file that cannot be converted returns [] (no fallback to a name search). Formal charges (including lone pairs on anions such as [O-]) and radical electrons are drawn.

How It Works

1. Input Recognition: Uses mol2chemfigPy3's automatic input type detection
2. ChemFig Generation: Converts to basic ChemFig code
3. Molecular Analysis: Parses structure with RDKit for geometry
4. Lone Pair Placement: Calculates and adds \charge{} annotations
5. Formatting: Produces clean, readable LaTeX output

Dependencies

• RDKit: Molecular structure processing and geometry
• mol2chemfigPy3: SMILES/InChI/CID to ChemFig conversion
• pubchempy: PubChem database queries (for formulas and names)

Examples

Basic Usage

from mol2lewis import lewis

# Water
print(lewis("O"))

# Benzene with aromatic circle
print(lewis("c1ccccc1", aromatic_circles=True))

# Ethanol from different sources
print(lewis("CCO"))           # SMILES
print(lewis(702))             # CID
print(lewis("ethanol"))       # Name

Advanced: Multiple Isomers

To get all structural isomers for a given molecular formula (e.g. $C_2H_6O$), use the option selection='all'

from mol2lewis import lewis

# List all isomers of C2H6O
isomers = lewis("C2H6O", selection="all")

for i, iso in enumerate(isomers, 1):
    print(f"Isomer {i}: {iso['iupac_name']} ({iso['name']})")
    print("Normal:\n", iso["normal"], "\n")
    print("Draft:\n", iso["draft"], "\n")

Each element in the isomers list is a dictionary with four keys:

• normal: chemfig code ready to use in LaTeX
• draft: annotated version to visualize lone pairs and charges
• iupac_name: IUPAC name of the molecule
• name: common name of the molecule

You can copy and paste these codes into a LaTeX document using the chemfig package (see above for details).

For draft mode, make sure to include the mol2lewis.sty package in your LaTeX preamble. It's a short file with only this inside (but draft mode can't work without it):

\ProvidesPackage{mol2lewis}[2025/01/22 v0.2.0 Commands for mol2lewis draft mode]
\RequirePackage{xcolor}

% Draft mode markers for bonds
\newcommand{\red}{\.[{.style={draw=red,fill=red}}]} % red dot for single bond in draft mode
\newcommand{\redd}{\:[{.style={draw=red,fill=red}}]}% red dots for double bond in draft mode
\newcommand{\draftlp}{\:}                           % two dots for lone pair in draft mode

\endinput

License

MIT License