I need to post it again because I forgot to add input files and the version number. Dear QE Users,
I am a PhD student working with Berry phase polarization calculations of single layer 2D material hBN in quantum espresso (applying vacuum in the z-direction). I found -0.0000002 C/m^2 polarization in the z-direction in Quantum ESPRESSO 7.0 without electric field; however, total phases belonging to electronic and ionic contribution found as equal to each other and given as 0.40381 and -0.40381 with 5 significant digits. Is it OK to take -0.0000002 C/m^2 as the polarization of the system? or Is this a numerical error? The material in the z-direction is not polar, is it possible that the polarization operator for a non-polar material is ill defined for this calculation? Should I use berry phase polarization for the polar materials only? Also, when I change the dimension of the unit cell in the z-direction and position of the material inside the cell, this polarization value changes even though I use the same material. Most importantly, this polarization value changes with the number of processors used with the mpirun -np. This is very strange, should I stick to -np 1. I should not make it parallel? I also saw that this polarization is given with microC/cm^2 in the papers. Does this mean the last two digits from " P = -0.0000002 (mod 2.9386420) C/m^2" are not considered from the results obtained in the Quantum ESPRESSO run? Are all those numerical errors or physical phenomena because I know that there are vacuum fluctuations and Lamb modes, and so on? Can we probe those with QE? I look forward to hearing from you soon. Best regards, Aykut Turfanda PhD student at Istanbul Technical University ############################### SUMMARY OF PHASES ~~~~~~~~~~~~~~~~~ Ionic Phase: -0.40381 Electronic Phase: 0.40381 TOTAL PHASE: -0.00000 MOD_TOT: 1 VALUES OF POLARIZATION ~~~~~~~~~~~~~~~~~~~~~~ The calculation of phases done along the direction of vector 3 of the reciprocal lattice gives the following contribution to the polarization vector (in different units, and being Omega the volume of the unit cell): P = -0.0000030 (mod 40.7880378) (e/Omega).bohr P = -0.0000000 (mod 0.0514003) e/bohr^2 P = -0.0000002 (mod 2.9386420) C/m^2 The polarization direction is: ( 0.00000 , 0.00000 , 1.00000 ) ################################ &control calculation = 'nscf', restart_mode='from_scratch' prefix = 'bn' outdir = './tempdir' pseudo_dir = '~/pseudo_my/' etot_conv_thr = 1d-05 forc_conv_thr = 1d-04 lberry=.true. gdir = 3 nppstr = 10 / &system ibrav = 0, nat = 2, ntyp = 2, ecutwfc = 80 occupations='fixed' smearing='gaussian' degauss=0.02 / &electrons conv_thr = 1d-08 / &ions / &cell / ATOMIC_SPECIES B 10.811 B.pbe.nc.sr.upf N 14.0067 N.pbe.nc.sr.upf ATOMIC_POSITIONS angstrom B -0.0000000000 1.4480800000 1.6085249900 N 1.2540740000 0.7240400000 1.6085249900 K_POINTS automatic 10 10 10 0 0 0 CELL_PARAMETERS angstrom 2.5081477165 0.0000000000 0.0000000000 -1.2540738583 2.1721196390 0.0000000000 0.0000000000 0.0000000000 21.5841000986 ############################## &control calculation = 'scf', restart_mode='from_scratch' prefix = 'bn' outdir = './tempdir' pseudo_dir = '~/pseudo_my/' etot_conv_thr = 1d-05 forc_conv_thr = 1d-04 / &system ibrav = 0, nat = 2, ntyp = 2, ecutwfc = 80 occupations='fixed' smearing='gaussian' degauss=0.02 / &electrons conv_thr = 1d-08 / &ions / &cell / ATOMIC_SPECIES B 10.811 B.pbe.nc.sr.upf N 14.0067 N.pbe.nc.sr.upf ATOMIC_POSITIONS angstrom B -0.0000000000 1.4480800000 1.6085249900 N 1.2540740000 0.7240400000 1.6085249900 K_POINTS automatic 10 10 1 0 0 0 CELL_PARAMETERS angstrom 2.5081477165 0.0000000000 0.0000000000 -1.2540738583 2.1721196390 0.0000000000 0.0000000000 0.0000000000 21.5841000986 #################################
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