Graphene band structure Matlab code

[Mohammad-gl]

I have this code for 3D band structure of graphene but i can't understand why while graphene first brillouin zone is hexagonal but here a rectangle form of first brillouin zone is used?E0 = 0; % coloumb integral
V = -2.7; % hopping integral [eV]
acc = 1.41; % c-c bond length [Angstrom]
lattice = acc*sqrt(3); % Lattice constant
% Creating necessary k-vectors
kx = linspace(-2*pi/(lattice),2*pi/(lattice),100);
ky = linspace(-2*pi/(lattice),2*pi/(lattice),100);
[k_x,k_y] = meshgrid(kx, ky);
% Energy values with the preset parameters

energy_mesh1 = (E0 + V*sqrt(1 + (4.*((cos(k_y/2*lattice)).^2)) + (4.*(cos(sqrt(3)/2*lattice*k_x)).*(cos(k_y/2*lattice)))));

energy_mesh2 = (E0 - V*sqrt(1 +(4.*((cos(k_y/2*lattice)).^2)) +(4.*(cos(sqrt(3)/2*lattice*k_x)).*(cos(k_y/2*lattice)))));% Plotting
surf(k_x, k_y, real(energy_mesh1));
hold on
surf(k_x, k_y, real(energy_mesh2));
colormap('jet');
shading interp
hx = xlabel('k_x');
hy = ylabel('k_y');
hz = zlabel('E(k)');

 

[Dr Transport]

plotting purposes only, the hexagonal part is in the energy bands themselves.