Single-layer graphene nearest-neighbor model
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t = 1.000000

Nearest-neighbor bond vectors:
d1 = (0.000000, 1.000000)
d2 = (0.866025, -0.500000)
d3 = (-0.866025, -0.500000)

Primitive reciprocal vectors:
b1 = (3.627599, 2.094395)
b2 = (-3.627599, 2.094395)

Hamiltonian:
H(k) = [[0, -t f(k)],
        [-t f*(k), 0]]

f(k) = sum_j exp(i k·d_j)
Band energies: E_±(k) = ± t |f(k)|

This nearest-neighbor model produces Dirac cones at K/K' and zero DOS at E = 0.