molscope 0.6.1

Lightweight molecular structure analysis, visualisation, graph export, and coarse-graining in Python.

MolScope

Lightweight molecular structure analysis, visualisation, graph export, and coarse-graining in Python. Read .xyz, .pdb, .cif and .sdf files (optionally gzip-compressed), select and analyse atoms, and visualise them in 3D. The .cif reader is a basic mmCIF parser for standard _atom_site coordinate loops, not a full mmCIF syntax implementation.

3D structure rendering	Residue contact map	Coarse-grained beads

What it does

• Read and write XYZ, PDB, mmCIF and SDF (gzip-aware), fetch structures by id from RCSB, and load multi-model NMR ensembles.
• Select and measure by chain, element or residue; compute distances, angles, dihedrals and Kabsch-aligned RMSD.
• Analyse centroids, radius of gyration, the inertia tensor, inferred bonds and contacts.
• Contact maps at atom or residue level, with heatmap plots.
• Ensembles: pairwise RMSD, RMSF, averaging, and conformer clustering.
• Export for ML: flat structural descriptors and molecular graphs for NetworkX, PyTorch Geometric and DGL.
• Coarse-grain onto residue, Martini-style or custom bead mappings.
• Visualise with 3D matplotlib plots, an interactive py3Dmol viewer, spin GIFs, and a command-line interface.

Install

With uv (recommended):

uv sync                     # creates .venv, installs deps + dev tools from the lockfile
uv run molscope 1fqy.pdb  # run the CLI
uv run pytest               # run the tests

uv sync pins the interpreter from .python-version and resolves against uv.lock for reproducible installs. Use uv sync --no-dev to skip the test tools.

With plain pip:

python -m venv .venv && source .venv/bin/activate
pip install -e ".[test]"    # or: pip install -r requirements.txt

Documentation

The documentation website is built with MkDocs Material:

uv sync --group docs
uv run mkdocs serve
python scripts/build_user_guide_pdf.py

Docs source lives in docs/; the site configuration is mkdocs.yml. The PDF builder requires Pandoc and a LaTeX engine such as xelatex.

Quickstart

A runnable end-to-end tour over the bundled sample structures lives in example.py:

uv run python example.py                  # opens 3D plot windows
MPLBACKEND=Agg uv run python example.py   # headless: saves PNGs instead

It reads an .xyz and a .pdb, prints derived properties, compares the NMR models of 1aml, writes a transformed structure back out, and renders a plot.

Library

import molscope as ms

mol = ms.read("1fqy.pdb")          # parser chosen from the extension
mol = ms.fetch("1fqy")             # ...or download straight from RCSB by id
print(mol.summary())                # atoms, formula, chains, bounding box

mol = mol.centered().rotate("z", 90).translate((1, 2, -1))
mol.plot()                          # CPK colours, inferred bonds, equal aspect

Molecule is immutable: translate, centered and rotate each return a new molecule, so transformations chain cleanly without aliasing. Equality is by value (np.array_equal on coordinates).

Selections

PDB files, and standard mmCIF atom-site loops, carry per-atom metadata (atom name, residue, chain), so you can slice a structure:

mol.select(chain="A")               # one chain
mol.select(element="C")             # all carbons
mol.select(resname="HOH")           # waters
mol.select(resid=(10, 20))          # an inclusive residue range
mol.alpha_carbons()                 # CA atoms (the usual basis for protein RMSD)
mol.backbone()                      # N, CA, C, O
mol[mask_or_indices]                # subset by numpy mask / index array

Analysis and measurements

mol.centroid, mol.center_of_mass    # geometric / mass-weighted centre
mol.radius_of_gyration              # compactness (angstrom)
mol.dimensions, mol.formula         # bounding box, Hill-order formula
mol.bonds()                         # inferred bond index pairs (KD-tree if scipy)
mol.contacts(cutoff=5.0)            # atom pairs within a distance

mol.distance(i, j)                  # bond length
mol.angle(i, j, k)                  # bond angle (degrees)
mol.dihedral(a, b, c, d)            # torsion angle (degrees)

a.alpha_carbons().rmsd(b.alpha_carbons(), align=True)   # CA-RMSD after Kabsch fit

Structural descriptors for ML

features = mol.descriptors()                 # flat dict of scalar/vector descriptors
features["radius_of_gyration"]
features["principal_moments"]                # 3 values
features["distance_histogram"]               # fixed-size histogram

X, names = ms.featurize_many(
    ["a.pdb", "b.pdb", "c.xyz"],
    return_names=True,
)                                            # numeric matrix + column names

Descriptors include atom/residue counts, element counts, molecular mass, centres, radius of gyration, bounding-box dimensions, inertia tensor, principal moments/axes, shape anisotropy, compactness, distance histograms, bond-length summary statistics, and atom/residue contact summaries. Full contact maps remain available through mol.contact_map(...).

Contact maps

cmap = mol.contact_map(cutoff=8.0, level="residue")   # CA-CA contacts -> ContactMap
cmap.matrix                                           # (R, R) array
mol.plot_contact_map(cutoff=8.0)                      # heatmap

mol.contact_map(level="atom")                         # atom-level map
mol.contact_map(level="residue", method="min")        # closest inter-residue atom
mol.contact_map(level="residue", method="com")        # residue centre of mass

NMR ensembles

from molscope import ensemble

models = ms.read_pdb_models("1aml.pdb")     # all 20 models
ensemble.rmsd_matrix(models)                 # pairwise RMSD matrix
ensemble.rmsf(models)                        # per-atom fluctuation
ensemble.average(models)                     # mean structure
ensemble.align_all(models)                   # superpose every model onto the first

# Per-residue-pair contact probability across the ensemble (NMR variability)
freq = ms.ensemble_contact_frequency(models, cutoff=8.0)
freq.plot()                                  # heatmap of contact frequencies in [0, 1]

Comparing and clustering conformers

Cluster an ensemble (NMR models, conformer sets, docking poses, MD snapshots) by pairwise RMSD:

matrix = ms.rmsd_matrix(models, align=True)        # (M, M) RMSD matrix
ms.plot_rmsd_heatmap(matrix)                        # heatmap

clusters = ms.cluster(models, method="hierarchical")   # data-driven cutoff
clusters = ms.cluster(models, n_clusters=3)            # ...or a fixed count
clusters.n_clusters                                  # how many clusters
clusters.groups()                                    # {cluster_id: [model indices]}
clusters.representatives()                            # {cluster_id: medoid model index}

ms.plot_rmsd_heatmap(matrix, order=clusters.order)  # reorder into diagonal blocks

Writing and viewing

ms.write_xyz(mol.centered(), "out.xyz")     # write transformed coordinates back
ms.write_pdb(mol, "out.pdb")

mol.plot(color_by="chain")                   # colour by element / chain / residue
mol.view(style="cartoon")                    # interactive py3Dmol viewer (notebooks)
from molscope.plotting import spin_gif
spin_gif(mol, "spin.gif")                    # rotating animation

Molecular graphs (for machine learning)

Turn 3D coordinates plus inferred bonds into a graph, then export to the common ML frameworks. The base to_graph() needs no extra dependencies; each exporter imports its backend lazily.

mol = ms.read("1fqy.pdb")

g = mol.to_graph()                  # MolecularGraph: nodes + edges, no deps
g.n_atoms, g.n_bonds                # counts
g.atomic_numbers, g.masses          # per-node arrays
g.node_features()                   # (N, 2) default features [atomic_number, mass]

G = mol.to_networkx()               # networkx.Graph with node/edge attributes
data = mol.to_pyg_data()            # torch_geometric.data.Data (x, pos, edge_index, edge_attr, z)
dglg = mol.to_dgl_graph()           # dgl.DGLGraph with ndata/edata tensors

Nodes carry element, atomic number, mass, coordinates and (from PDB/mmCIF) atom name, residue and chain. Edges carry the bonded pair, interatomic distance, and bond order (1.0 for geometrically inferred bonds). Install backends as needed: pip install "molscope[graph]" installs only NetworkX. PyTorch Geometric and DGL are optional manual installs: pip install torch torch_geometric or pip install dgl after choosing the right PyTorch build for your platform.

Coarse-graining

Map an atomistic structure onto a smaller set of beads. The result is an ordinary Molecule (beads as "atoms") with explicit CG bonds attached, so it plots, transforms and graphs like anything else.

mol = ms.read("1fqy.pdb")

cg = mol.coarse_grain("residue_com")        # one bead per residue (centre of mass)
cg = mol.coarse_grain("residue_centroid")   # ...or geometric centroid
cg = mol.coarse_grain("martini")            # simplified backbone + side-chain beads
cg.plot(scale=200)                          # beads + backbone topology
print(cg.mapping_report())                  # explain beads, dropped atoms, and bonds

# Custom bead definitions by residue + atom name (needs PDB/mmCIF metadata)
mapping = {"ALA": {"BB": ["N", "CA", "C", "O"], "SC": ["CB"]}}
cg = mol.coarse_grain(mapping)
cg, report = mol.coarse_grain(mapping, return_report=True)

# Custom bead definitions by atom index (works on ANY structure, even .xyz)
cg = mol.coarse_grain({"head": [0, 1, 2, 3], "tail": [4, 5, 6, 7]},
                      bonds=[("head", "tail")])   # define the bead network too

cg.to_graph()                               # CG bead network, ready for ML

Bead positions are mass-weighted (or centroids). For residue mappings bonds are generated automatically (within a residue, plus a backbone chain between residues); pass bonds= to define them yourself. Name-based bonds are intended for unique bead names such as head/tail; repeated names such as BB/SC are ambiguous, so use bead indices for those. Atoms you leave unassigned are dropped with a warning. This is meant for teaching and prototyping CG mappings, not as a replacement for production Martini parameters.

Command line

molscope helix_201.xyz --translate 1 2 -1
molscope 1fqy.pdb --select atom_name=CA --color-by residue --save ca.png
molscope --fetch 1aml --center --gif amyloid.gif
python -m molscope 1fqy.pdb          # equivalent if not pip-installed

Sample structures

File	Contents
helix_201.xyz	a helix (bare coordinates)
1fqy.pdb	Aquaporin-1, single model (1661 atoms)
1aml.pdb	Alzheimer amyloid A4 peptide, 20-model NMR ensemble

Notes

• PDB files are parsed by fixed columns, not whitespace splitting, so atoms with touching coordinate fields (large or negative values) read correctly.
• Alternate conformations (altLoc) other than the primary one are skipped.
• read_pdb returns a single model (model=1 by default); use read_pdb_models for the whole ensemble.
• Bond inference uses a scipy.spatial.cKDTree when available; without scipy it falls back to a dense O(n^2) search that is refused above ~8000 atoms.
• Optional extras: pip install "molscope[fast]" (scipy, faster bonds/contacts) and "molscope[viz]" (py3Dmol, for Molecule.view).

Tests and linting

uv run pytest          # full test suite
uv run ruff check .    # lint

CI (GitHub Actions) runs both across Python 3.9 / 3.11 / 3.13 on every push and PR.

License

MIT