Dear QE Community, I have compiled qe-6.0 with intel-14 and openmpi-1.6.5. In attempting to speed up the phonon calculation (as suggested in other posts), I have tried two different alpha_mix(1) values- the default value of 0.7 and 0.3. For Si, the two alpha_mix values give almost identical phonon frequencies for all q-points. For GaAs, I get different frequencies. I suspect it may be due to the LO-TO splitting at the zone center but the differences exist for other q-points as well particularly for the acoustic modes.

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For example, for q=(0,0,0), I get the following frequencies in wavenumber in the .dyn files: alpha_mix(1)=0.7 : 5.936331, 5.936331, 5.936331, 249.858731,249.858731,249.858731 alpha_mix(1)=0.3 : -17.197390, -17.197390,-17.197390, 247.538697, 247.538697, 247.538697 For q=(0.125,-0.125,0.125) I get: alpha_mix(1)=0.7 : 31.681603, 31.681603, 62.388768, 246.723626, 246.723626, 266.652186 alpha_mix(1)=0.3 : 34.440636, 34.440636, 62.925154, 247.034644, 247.034644, 266.813369 For q=(-0.25,0.25,-0.25) I get: alpha_mix(1)=0.7 : 49.997159, 49.997159, 120.235286, 242.448112, 242.448112, 255.746902 alpha_mix(1)=0.3 : 52.457875, 52.457875, 121.042662, 242.645430, 242.645430, 255.836026 The issue of negative phonon frequencies has been exhaustively discussed on the forum and is not the point of my post. I assume that alpha_mix is not a convergence parameter and should not change the frequencies but it does. I have also run on SnSe where I do not get negative frequencies at all for either alpha_mix values but the frequencies differ by 10-20 wavenumbers between the two sets. I should add that all self-consistent calculations for the individual modes reached convergence. Convergence tests were done for the cutoff energy and K-grid as well. Here are my input files for the initial self-consistent run followed by the phonon calculation: ============================ &control calculation = 'scf' prefix = 'gaas' restart_mode = 'from_scratch' wf_collect = .false. pseudo_dir = './' outdir = './' verbosity = 'high' tprnfor = .true. tstress = .true. / &system ibrav=2,celldm(1)=10.838928, nat= 2, ntyp= 2 , nbnd=16 ecutwfc = 70, ecutrho=280, occupations = 'fixed' / &electrons diagonalization = 'david' mixing_beta = 0.7 conv_thr = 1.0d-10 / ATOMIC_SPECIES Ga 69.723 Ga.pbe-hgh.UPF As 74.92160 As.pbe-hgh.UPF ATOMIC_POSITIONS crystal Ga 0.0 0.0 0.0 As -0.25 0.75 -0.25 K_POINTS automatic 12 12 12 0 0 0 ============================== GaAs &inputph tr2_ph = 1.0d-12 prefix = 'gaas' amass(1) = 69.723 amass(2) = 74.92160 ldisp = .true. epsil = .true. nq1=8, nq2=8, nq3=8 outdir = './' alpha_mix(1)=0.3 fildyn = 'gaas.dyn' fildvscf = 'dvscf' / =============================== I would appreciate any input as to why alpha_mix alters the results for polar materials such as GaAs and SnSe. Thank you, Vahid Vahid Askarpour Department of Physics and Atmospheric Science Dalhousie University, Halifax, NS, Canada

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