Dear all Quantum Espresso users, I have some questions regarding to structure relaxation on a charged system using smearing.
Objective: The system I am interested in is a molecule in a cubic cell with large vacuum space. I would like to perform structure relaxation, then use the optimized atomic coordinate to perform self-consistent calculation and phonon calculation for Raman Spectrum using the flag Iraman = .true.. The system has 3- charge, so I used tot_charge = -3. Problem: In the structure relaxation calculation, I found that when I used the flag occupations = 'fixed', the energy didn't converged after 200 cycles. By contrast, when I used occupations = 'smearing', the energy converged without issues. I also found that occupations = 'fixed' and occupations = 'smearing' both work in the self-consistent calculation. 1. My first question is whether I need to use the same option of occupations (occupations = 'fixed' or occupations = 'smearing') for both structure relaxation and the self-consistent calculation. 2. My second question is that what kind of output in the output file would provide a hint on why the energy didn't converged in the structure relaxation calculation when using occupations = 'fixed'. Below is my input file for relaxation: ------------------------------------------------------------------------------------------------------ &CONTROL calculation = "relax", prefix = "ttc", pseudo_dir = "/home/weilunt/pseudopotential/norm_conserving/pz", outdir = "./tmp", / &SYSTEM ibrav = 1, celldm(1) =25.0, nat = 9, ntyp = 3, ecutwfc = 120.D0, tot_charge = -3, occupations = 'fixed', ! occupations = 'smearing', ! degauss = 0.02, / &ELECTRONS conv_thr = 1.D-8, mixing_beta = 0.7, electron_maxstep = 200, / &IONS / ATOMIC_SPECIES S 32.070 S.pz-hgh.UPF N 14.007 N.pz-hgh.UPF C 12.010 C.pz-hgh.UPF ATOMIC_POSITIONS (angstrom) S 0.0000 -1.2375 0.0000 S -1.4289 1.2375 0.0000 S 1.4289 1.2375 0.0000 N 0.7145 0.0000 0.0000 N -0.7145 0.0000 0.0000 N 0.0000 1.2375 0.0000 C 0.0000 -0.4125 0.0000 C -0.7145 0.8250 0.0000 C 0.7145 0.8250 0.0000 K_POINTS (automatic) 1 1 1 0 0 0 ---------------------------------------------------------------------------- 3. I performed the self-consistent calcualtion using nbnd = 40 at the Γ point for both occupations = 'fixed' and occupations = 'smearing' and compared the band energy. I found that the band energy are quite different in these two cases. Are they supposed to have huge difference? Below is the output for these two cases: ---------------------------------------------------------------------------- occupations = 'fixed' k = 0.0000 0.0000 0.0000 (189047 PWs) bands (ev): -21.3134 -18.2350 -18.2296 -14.3475 -14.3259 -13.4357 -10.5406 -10.5270 -9.6984 -7.4329 -6.8305 -5.2388 -5.2188 -4.7928 -4.7839 -4.4779 -2.2839 -2.2721 -1.7952 -1.7650 -1.7571 -1.4256 -1.3879 -1.3618 -1.0115 -0.1342 -0.0938 -0.0671 -0.0246 -0.0065 0.3019 0.7790 0.8220 0.8520 0.8662 0.8663 0.9333 0.9548 1.0616 1.2307 highest occupied, lowest unoccupied level (ev): -1.3618 -1.0115 ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- occupations = 'smearing' k = 0.0000 0.0000 0.0000 (189047 PWs) bands (ev): -43.8773 -33.1465 -33.1425 -30.7223 -29.4208 -29.4170 -28.8619 -28.0248 -21.0563 -21.0516 -17.6879 -17.6845 -10.4974 -10.0052 -10.0041 -7.0735 -7.0715 -5.6177 -5.6166 -5.1731 -4.8122 -3.1190 -1.6271 -1.2365 -1.1549 -1.1531 -1.0476 -1.0205 -0.7347 -0.6855 -0.6690 -0.6576 -0.6294 -0.3364 -0.0832 0.1631 0.1694 0.1843 0.2041 0.2461 the Fermi energy is -1.2926 ev ---------------------------------------------------------------------------- Thank you very much for your help and time in advance, Weilun Tang Department of Materials Science and Engineering Iowa State University
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