Dear Sahu, The KPM method is based on a probabilistic algorithm. Each time you run the code the set of energies that you get is different. Since, you are calculating the conductance at the energies provided by the KPM method, you will of course get different values for each run. In different words: rho = kwant.kpm.SpectralDensity(model1) eng1, densities = rho.energies, rho.densities eng1 is different between two different runs of the program. I hope this helps, Adel On Tue, Dec 13, 2022 at 9:18 AM <sahu.ajit92@gmail.com> wrote:

Dear All, I am using Kwant to calculate the conductance of a Graphene structure. However, it gives different conductance each time I run the code. I didn't change the code. For the same code, I am getting different results each time. Is there some error in my code or am I using the kwant function incorrectly? Please Help

Code ----------- import kwant import matplotlib.pyplot as plt

Wnr = 99 lnr = 60 a = 2.46 lat = kwant.lattice.honeycomb(a,norbs=1) a,b = lat.sublattices

def make_sys(): def rect(pos): x,y = pos return 0<=x<=lnr and 0<=y<=Wnr

def delet(pos): x, y = pos return 0<=x<=35 and 20<=y<=80

model1 = kwant.Builder() model1[lat.shape(rect,(1,1))] = 0 model1[lat.neighbors()] = 2.64 del model1[lat.shape(delet, (11,25))] model1.eradicate_dangling() return model1

def lead_attach(model1):

def lead1_shape(pos): x, y = pos return 0<y<=20

def lead2_shape(pos): x, y = pos return 80<=y<=Wnr

sym = kwant.TranslationalSymmetry(lat.vec((-1,0))) #from the left sym.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) lead = kwant.Builder(sym) lead[lat.shape(lead1_shape, (1,2))]=0 lead[lat.neighbors()] =2.64 model1.attach_lead(lead)

########### lead 2 #################

sym1 = kwant.TranslationalSymmetry(lat.vec((-1,0))) #from the left sym1.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym1.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) lead2 = kwant.Builder(sym1) lead2[lat.shape(lead2_shape, (2,82))] = 0 lead2[lat.neighbors()] = 2.64 model1.attach_lead(lead) model1.attach_lead(lead2) kwant.plot(model1) return model1

def dos_eng(model1): rho = kwant.kpm.SpectralDensity(model1) eng1, densities = rho.energies, rho.densities return eng1, densities

def calc_conductance(sys, energies): data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0))

plt.figure() plt.plot(energies, data) plt.xlim([-3,3]) plt.ylim([0,4]) plt.xlabel("Energy (eV)",fontweight='bold') plt.ylabel("Conductance $(e^2/h)$",fontweight='bold') return data

def main(): sys = make_sys() syst = lead_attach(sys) syst = syst.finalized() eng1, densities = dos_eng(syst) cond = calc_conductance(syst,eng1)

if __name__ == '__main__': main()

-- Abbout Adel