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small residule charges after SCF is completed Date: 2014/05/29 04:19 Name: Chen Dear Prof. Ozaki, I am now using Openmx to do DFT/NEGF calculations for Fe/MgO/Fe junction. I noticed one feature of DFT/NEGF calculations of Openmx and I not sure this is a general feature for DFT/NEGF calculation. After my SCF iteration is completed, i.e. the convergence accuracy is reached, I usually checked the total charge and dipole moment. For total charge, I got "total= 168.04573 ideal(neutral)= 168.00000". For dipole moment, I got " Absolute D 0.73555550 ". For total charge, I know it is hard to reach the ideal charge since we are handling a open and nonequilibrium system. But, in my case, the difference between calculated charge and ideal charge, i.e. 0.04573 here, is a little bit large. Even when "NormRD" is very small, e.g. <1.0E-10, such a difference of 0.04573 can not be reduced any more. I am wondering if we can make it as small as possible (<0.001). For the dipole moment, I have the same concern with it. Although I can do something for the structure, for example making the system more symmetrical, I usually get a moderate dipole moment and can not make it smaller. Do you have any suggetions? Thanks a lot. Y. Chen

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Re: small residule charges after SCF is completed ( No.1 ) Date: 2014/05/29 21:44 Name: T. Ozaki Hi, The chemical potentials for left and right leads are fixed during the NEGF calculations. So there is no theoretical reason for conservation of the charge in general cases. An exceptional case is a perfect system that the scattering part consists of exactly the same materials as the leads. In this case, the chemical potential of the central part is the same as those of the leads, thus leading to charge conservation. The dipole moment along periodic bulk direction does not have physical meaning as explained in Berry phase theory. However, if you calculate a slab model having vacuum region, the dipole moment perpendicular to the slab does have physical meaning. Regards, TO Re: small residule charges after SCF is completed ( No.2 ) Date: 2014/05/30 22:13 Name: Chen Dear Prof. Ozaki, Thank you for your explanation. Chen Re: small residule charges after SCF is completed ( No.3 ) Date: 2014/06/10 09:24 Name: Chen Dear Prof. Ozaki, I still have one question about origin of dipole moment. My system is Fe/MgO/Fe junction. I did BAND calculations for left and right leads and then did NEGF calculations for the junction. For the Band calculation of Fe leads, I checked the dipole moment and found the dipole is very small (0.00029668 Debye). This is expected because the system is symmetric and no dopants are present. For the NEGF calculation of Fe/MgO/Fe junction, as I mentioned in the previous question, the dipole moment is about 0.7355 Debye. Please see the below for detailed information of the dipole moment. In general, the dipole could be arisen from two reasons. The first one is the asymmetric structure (e.g. ZnO, TiO2) and the second is arisen from doping in the system (e.g. p-n junction). According to these argument, I checked the system carefully. I found my system is a completely symmetric structure along x,y,z directions. Secondly, there is no dopant in my system. In the below, I found the positive charge along x direction is about 2453.49724549 but negative one 2454.22263332. I am wondering what other reasons could explain this dipole moment in openmx calculation if there is no asymmetry and no dopants. Thank you in advance for your further explanation. Best, Chen /****************output from NEGF calculation************/ ******************************************************* Dipole moment (Debye) ******************************************************* Absolute D 0.73555550 Dx Dy Dz Total -0.72538783 -0.08618118 -0.08618117 Core 2453.49724549 578.17160875 578.17160875 Electron -2454.22263332 -578.25778993 -578.25778992 Back ground -0.00000000 -0.00000000 -0.00000000 ******************************************************* Re: small residule charges after SCF is completed ( No.4 ) Date: 2014/06/10 18:39 Name: Artem  <artem.pulkin@epfl.ch> "the system is symmetric" Are the leads also mirror-symmetric with respect to the scattering region? Re: small residule charges after SCF is completed ( No.5 ) Date: 2014/06/15 12:35 Name: Chen Dear Artem, The below, I present the calculated dipole moment and the input structure of NEGF calculations. Could you please tell me if there is mirror symmetry? Thanks a lot for your help. Chen ******************************************************* Dipole moment (Debye) ******************************************************* Absolute D 11.77731099 Dx Dy Dz Total -0.94575796 -8.30092465 -8.30091839 Core 3543.94046571 619.46958080 619.46958080 Electron -3544.88622367 -627.77050545 -627.77049919 Back ground -0.00000000 -0.00000000 -0.00000000 ******************************************************* **********************Input structure******************** LeftLeadAtoms.Number 4 <LeftLeadAtoms.SpeciesAndCoordinates # Unit=Ang. 1 Fe -6.459 0.000 0.000 8.000 6.000 2 Fe -5.026 1.433 1.433 8.000 6.000 3 Fe -3.593 0.000 0.000 8.000 6.000 4 Fe -2.160 1.433 1.433 8.000 6.000 LeftLeadAtoms.SpeciesAndCoordinates> Atoms.Number 10 <Atoms.SpeciesAndCoordinates 1 Mg 0.000 0.000 0.000 4.000 4.000 2 O 0.000 1.433 1.433 3.000 3.000 3 Mg 2.027 1.433 1.433 4.000 4.000 4 O 2.027 0.000 0.000 3.000 3.000 5 Mg 4.054 0.000 0.000 4.000 4.000 6 O 4.054 1.433 1.433 3.000 3.000 7 Mg 6.081 1.433 1.433 4.000 4.000 8 O 6.081 0.000 0.000 3.000 3.000 9 Mg 8.108 0.000 0.000 4.000 4.000 10 O 8.108 1.433 1.433 3.000 3.000 Atoms.SpeciesAndCoordinates> RightLeadAtoms.Number 4 <RightLeadAtoms.SpeciesAndCoordinates # Unit=Ang. 1 Fe 10.268 1.433 1.433 8.000 6.000 2 Fe 11.701 0.000 0.000 8.000 6.000 3 Fe 13.134 1.433 1.433 8.000 6.000 4 Fe 14.567 0.000 0.000 8.000 6.000 RightLeadAtoms.SpeciesAndCoordinates> Re: small residule charges after SCF is completed ( No.6 ) Date: 2014/06/16 15:24 Name: T. Ozaki Hi, > Could you please tell me if there is mirror symmetry? Thanks a lot for your help. One can see a mirror symmetry along the x-axis, while there is no mirror symmetry along y- and z-axes. Regards, TO

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