Dear Sir,
  I am a beginner user of kwant i want to make a ring shaped structure of bi2se3 .it is rhombohedral having lattice constants a = 4.138, c = 28.64.
Internal coordinates u = 0.399 for Bi sites v= 0.206 for Se sites.
The basic unit cell of Bi2Se3 is called Quituple layer which contains 5 atoms, one Se atom at (0,0,0), Bi atoms at internal coordinates(+_u,+_,u+_u) and Se atoms at (+-v,+-v,+-v)
The Bravis lattice vectors are
a1 = (-a/2,-a/2*sqrt(3),c/3)
a2 = (a/2,-a/2*sqrt(3),c/3)
a3=(0,-a/sqrt(3), c/3)

for bi2Se3 i wrote the following program..Please tell whether it is correct or not and please tell how to form a ring structure of bi2se3

from math import pi, sqrt, tanh
import kwant
# For computing eigenvalues
import scipy.sparse.linalg as sla

# For plotting
from matplotlib import pyplot
# Define bismuth selenide lattice
a= 4.138
c= 28.64
u= 0.399
v= 0.206
bismuthselenide = kwant.lattice.general([(-4.138/2,-4.138/2*sqrt(3),28.64/3),(4.138/2,-4.138/2*sqrt(3),28.64/3),(0,-4.138/sqrt(3),28.64/3)],
[(0, 0, 0),(0.399, 0.399, 0.399),(0.206, 0.206, 0.206)])
a, b, c = bismuthselenide.sublattices