#!/usr/bin/env python
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at https://mozilla.org/MPL/2.0/.

# This example creates the tight-binding Hamiltonian
# for graphene with on-site energy 0, and hopping energy
# -2.7 eV.

import sisl

bond = 1.42
# Construct the atom with the appropriate orbital range
# Note the 0.01 which is for numerical accuracy.
C = sisl.Atom(6, R = bond + 0.01)
# Create graphene unit-cell
gr = sisl.geom.graphene(bond, C)

# Create the tight-binding Hamiltonian
H = sisl.Hamiltonian(gr)
R = [0.1 * bond, bond + 0.01]

for ia in gr:
    idx_a = gr.close(ia, R)
    # On-site
    H[ia, idx_a[0]] = 0.
    # Nearest neighbour hopping
    H[ia, idx_a[1]] = -2.7

# Calculate eigenvalues at K-point
print(H.eigh([2./3, 1./3, 0.]))
