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