Dear QE users, I just noticed that pw.x was giving me different energy values. I don't know if it's just due to machine accuracy or something strange happens.
1) I did the scf calculation with k points generated automatically and got energy_1 (-362.64960323 Ry). The input file is attached at the end of this email. 2) I copy the k points and weights spit out by step 1) to the input file and redo the calculation, I got exactly the same value as energy_1. 3) I add nosym=.true. in the input and redo the calculation. I got energy_3 (-362.64951123 Ry) which is different but not far from energy_1. The difference is 0.00009200 Ry Is this normal? Then I tried to do a calculation with hybrid functional (HSE). q-mesh 1x1x1. the same step 1) I got energy energy_hybrid_1 (-362.47024611 Ry)_ the same step 2) I got the same energy value as energy_hybrid_1 the same step 3) I got energy_hybrid_3 (-362.46997237 Ry) Now the difference is 0.00027374 Ry about 3 times as the previous one. Is this normal? For my understanding, nosym affects the k point generation. I checked the output of 1) 2) 3) and they have the same kpoints and weights. In principle, I thought step 2) and 3) should yield the same energy. Only machine accuracy plays a little bit. If nosym is doing more than K points, please bear my limited knowledge and explain me where else symmetry is also involved and which might be relevant to my calculation. Let me know your thoughts. Thanks! Ye &control calculation = 'scf' restart_mode='from_scratch', prefix='TiO2-rutile', tstress = .true. tprnfor = .true. pseudo_dir = '/home/yeluo/pw_pseudo/', outdir='./out', / &system ibrav = 0, nat = 6, ntyp = 2, ecutwfc = 300, input_dft = 'pbe' lda_plus_u = .true., Hubbard_U(1) = 4.86041 / &electrons mixing_beta = 0.7 conv_thr = 1.0d-10 / CELL_PARAMETERS bohr 8.681023246 0 0 0 8.681023246 0 0 0 5.59094331 ATOMIC_SPECIES Ti 1.0 Ti.opt.upf O 1.0 O.ncpp ATOMIC_POSITIONS crystal Ti 0.0000000 0.0000000 0.0000000 Ti 0.5000000 0.5000000 0.5000000 O 0.3050990 0.3050990 0.0000000 O 0.6949010 0.6949010 0.0000000 O 0.1949010 0.8050990 0.5000000 O 0.8050990 0.1949010 0.5000000 K_POINTS automatic 8 8 8 1 1 1 =================== Ye Luo, Ph.D. Leadership Computing Facility Argonne National Laboratory
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