# transiesta does not converge for biased system

Dear Siesta users,

I am running transiesta on a metal/heavily p-doped semiconductor/metal system. At zero bias transiesta converges well according to the following convergence criteria

SCF.Mix density

SCF.DM.Converge .true.

SCF.DM.Tolerance 1.d-4

SCF.H.Converge .true.

SCF.H.Tolerance 1.d-3 eV

However, as soon as there is a non-zero bias I am unable to converge, even for very small values for the bias, say 0.1 eV. I tried different values for mixing and different grid spacing for the equilibrium and non-equilibrium parts. I have also controlled the variation of the Fermi level. Nothing helped. The convergence does make progress, but stops short of the criteria above. For example, using SCF.Mix density I can only reach an error of about 7e-3 for the density matrix while for the hamiltonian it does much better:

ts-scf: 82 -148924.176493 -148924.299263 -148924.299263 0.007402 -3.579231 0.000068

ts-scf: 83 -148924.177217 -148924.300175 -148924.300175 0.007312 -3.579231 0.000209

ts-scf: 84 -148924.178657 -148924.300099 -148924.300099 0.007197 -3.579231 0.000103

If I continue the calculation above the total energy keeps oscillating in the third decimal value. If I use instead SCF.Mix hamiltonian, then the density matrix converges well but the hamiltonian does not:

ts-scf: 44 -148924.261238 -148924.250726 -148924.250726 0.003672 -3.579231 0.022974

ts-scf: 45 -148924.247314 -148924.248288 -148924.248288 0.003802 -3.579231 0.018166

ts-scf: 46 -148924.225513 -148924.233580 -148924.233580 0.002878 -3.579231 0.028566

ts-scf: 47 -148924.269291 -148924.256242 -148924.256242 0.000828 -3.579231 0.007620

ts-scf: 48 -148925.094886 -148924.955382 -148924.955382 0.011201 -3.578819 0.369875

ts-scf: 49 -148924.966051 -148924.962300 -148924.962300 0.000209 -3.578819 0.376305

ts-scf: 50 -148924.941578 -148924.948857 -148924.948857 0.000210 -3.578819 0.372517

Here the oscillation is in the first decimal value. I have done many biased calculations before without this problem. Can anybody help with some ideas on how to control these oscillations to reach the desired convergence criteria?

Thanks a lot, Leo

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