Periodic systems

vibe-qc supports 1D, 2D, and 3D periodicity through a single PeriodicSystem type. Dimensionality is specified by dim ∈ {1, 2, 3}; the lattice matrix is always a full-rank 3×3.

import numpy as np
from vibeqc import Atom, PeriodicSystem

# 1D chain — column 0 of lattice is the lattice vector, the other two
# columns are implicit vacuum axes with enough separation to decouple
# images.
sysp_1d = PeriodicSystem(
    dim=1,
    lattice=np.diag([4.0, 30.0, 30.0]),
    unit_cell=[Atom(1, [0, 0, 0]), Atom(1, [0, 0, 1.4])],
)

# 2D slab — columns 0 and 1 are in-plane lattice vectors, column 2 is
# vacuum perpendicular to the sheet.
sysp_2d = PeriodicSystem(
    dim=2,
    lattice=np.diag([2.5, 2.5, 30.0]),
    unit_cell=[Atom(6, [0, 0, 0])],
)

# 3D bulk — all three columns are lattice vectors.
sysp_3d = PeriodicSystem(
    dim=3,
    lattice=np.diag([5.4, 5.4, 5.4]),       # Si conventional cubic, bohr
    unit_cell=[Atom(14, [0, 0, 0]), Atom(14, [0.25, 0.25, 0.25])],
)

Accessors

sysp.reciprocal_lattice()      # 3×3 matrix, columns = b_i with a_i · b_j = 2π δ_ij
sysp.n_electrons()             # total electrons per unit cell (Z - charge)
sysp.unit_cell_molecule()      # Molecule view for BasisSet() construction

Space-group analysis

from vibeqc import attach_symmetry
attach_symmetry(sysp)           # populates sysp.symmetry via spglib
print(sysp.symmetry)
# SpaceGroup(number=227, symbol='Fd-3m', order=48)

SpaceGroup carries number (International Tables), international_symbol, hall_number, point_group, and the full list of symmetry operations.

POSCAR I/O

from vibeqc import read_poscar, write_poscar

sysp = read_poscar("tests/data/NaCl.poscar")   # returns Crystal
# To convert between Crystal and PeriodicSystem, see the next section.