Nanostructures¶

Nanotubes and graphene sheets are built natively (the structure builders) and re-exported by MolPy. CarbonTubeBuilder rolls an exact graphene lattice into a single-wall carbon nanotube; GrapheneBuilder emits a flat honeycomb sheet. The tube's shape is fixed when the builder is constructed; unit-cell enumeration, seam closure, and connectivity are implementation details.

Zigzag, armchair, and chiral tubes¶

The usual (n, m) indices select the topology:

from molpy.builder import CarbonTubeBuilder, GrapheneBuilder

zigzag = CarbonTubeBuilder(8, 0, length=30.0).build()
armchair = CarbonTubeBuilder(6, 6, cells=4).build()
chiral = CarbonTubeBuilder(6, 3, cells=3).build()

sheet = GrapheneBuilder(8, 8, periodic_xy=True).build()

length rounds up to complete translational cells. Use cells when the exact number of cells matters; the two arguments are mutually exclusive. All three tubes are open along the axis and have dangling end valences.

Periodic tubes¶

Set periodic=True to close the axial bonds. build() returns the molecular graph; cell() returns the simulation cell those coordinates were laid out in, with only the z axis periodic:

builder = CarbonTubeBuilder(10, 10, length=50.0, periodic=True)
periodic = builder.build()
box = builder.cell(vacuum=12.0)

assert box.pbc.tolist() == [False, False, True]
assert all(len(periodic.get_neighbors(atom)) == 3 for atom in periodic.atoms)

The two are separate products because a molecular graph is topology and chemistry — the cell describes the simulation, and its one home is frame.box:

frame = periodic.to_frame()
frame.box = box

The circumference is part of the molecular topology, not a simulation-box periodic direction. Bonds come from the rolled graphene lattice rather than a Cartesian distance cutoff, so the seam is exact for zigzag, armchair, and general chiral tubes.

Atom annotations and deferred topology¶

The scalable default creates atoms and bonds only. Per-atom data can be written at build time, while angles and dihedrals remain optional:

atoms_only = CarbonTubeBuilder(8, 0, cells=20).build(atom_type="CA", charge=0.0)
with_topology = CarbonTubeBuilder(8, 0, cells=2).build(finalize="topology")

assert not list(atoms_only.angles)
assert list(with_topology.angles)
assert list(with_topology.dihedrals)

For a very large tube, keep the atoms-only graph through construction and let the MD export workflow materialize higher-order topology when it is actually needed.