Hi Ted, One possibility is that a fixed set of G-vector is selected using the initial cell based on the specified `ecutwfc`. This G-vector set is kept fixed during a vc-relax run (which can modify effectively the stored G-vectors through changes in the unit cell). As such, at the end of vc-relax, the planewave cutoff can differ from direct scf calculation if the unit cell undergoes sizable strain. One solution to this problem is to use the approach by M. Bernasconi et al, [J. Phys. Chem. Solids 56, 501 (1995), doi:10.1016/0022-3697(94)00228-2]. Relevant QE input parameters include: ecfixed, qcutz, and q2sigma see: https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm406
Hope that helps. Hsin-Yu -- Hsin-Yu Ko Postdoctoral Research Fellow Department of Chemistry and Chemical Biology Cornell University ________________________________ From: users <[email protected]> on behalf of 杨腾 <[email protected]> Sent: Sunday, June 19, 2022 1:51 AM To: [email protected] <[email protected]> Subject: [QE-users] Stress values from vc-relax and scf are different, but why? Dear QE users and experts, I am pretty confused by the different outputed stress value from both the vc-relax (the last step) and scf steps. Could you please help me to figure out why. Thank you so much! Ted Here is the output stress from the last step of vc-relax: total stress (Ry/bohr**3) (kbar) P= 0.01 0.00000028 -0.00000000 -0.00000013 0.04 -0.00 -0.02 -0.00000000 -0.00000011 -0.00000000 -0.00 -0.02 -0.00 -0.00000013 -0.00000000 0.00000011 -0.02 -0.00 0.02 and the output stress from scf: total stress (Ry/bohr**3) (kbar) P= -417.74 -0.00286693 0.00000000 -0.00006361 -421.74 0.00 -9.36 0.00000000 -0.00279769 0.00000000 0.00 -411.55 0.00 -0.00006361 0.00000000 -0.00285453 -9.36 0.00 -419.92 The input files for vc-relax is as below: ---------------start of vc-relax.in--------------------- &CONTROL calculation = 'vc-relax' verbosity = 'high' restart_mode = 'from_scratch' wf_collect = .true. nstep = 200 tstress = .true. tprnfor = .true. outdir = './' prefix = 'NiP2-monoclinic' etot_conv_thr = 1.0D-6 forc_conv_thr = 1.0D-5 pseudo_dir = '../../pp/' !tefield = .true. !add saw-like potential !dipfield = .true. !lelfield = .true. !nberrycyc = 5 !gdir = 3 !nppstr = 1 / &SYSTEM ibrav = 0 celldm(1) = 1.6896 !celldm(2) = !celldm(3) = 9.5983431328106 nat = 12 ntyp = 2 !nbnd = !tot_charge = !tot_magnetization = !starting_magnetization(1) = !angle1(1) = !angle2(1) = ecutwfc = 120 ecutrho = 480 !if ncpp,stick to the 4* relation !nr1 = !nr2 = !nr3 = !nosym = .true. !noinv = .true. !no_t_rev = .true. ! disable the usage of magnetic symmetry operations !occupations = 'fixed' ! set to 'tetrahedra' if calculate dos occupations = 'smearing' smearing = 'gaussian' degauss = 0.01 ! check the smearing contribution to total energy and if it ! is large then try to lower the value nspin = 1 ! 1:non-polarized 2: magnetization along z axis !noncolin = .true. ! magnetization in generic direction, !lspinorb = .true. ! soc calculation use a pseudopotential with spin-orbit. !assume_isolated= '2D' !input_dft = 'vdW-DF' ! defining the DFT functional !nqx1 = 1 ! proportional to nk1; for hybrid functions !nqx2 = 1 ! proportional to nk2 !nqx3 = 1 ! proportional to nk3 !lda_plus_u = .true. !Hubbard_U(1) = 0 !Hubbard_U(2) = 0 !vdw_corr = 'DFT-D' ! Dispersion correction in vdw calculations !edir = 3 ! This is the direction of applied field !emaxpos = 0.95 !eopreg = 0.1 !eamp = 0.019446905 ! Amplitude of e-field 1a.u. = 51.4220632*10^10 V/m / &ELECTRONS electron_maxstep = 1000 conv_thr = 1.0D-10 mixing_mode = 'plain' !mixing_mode = 'local-TF' mixing_beta = 0.5 diagonalization = 'david' !diago_thr_init = 1.0D-13 ! for non-scf calculations !diago_full_acc = .true. !efield = 0.027502070 ! 1 a.u. = 36.3609*10^10 V/m !efield_cart(1) = 0.0 !efield_cart(2) = 0.0 !efield_cart(3) = 0.027502070 !startingpot = 'file' !start from existing charge file !startingwfc = 'file' / &IONS ion_dynamics = 'bfgs' upscale = 1.0D3 trust_radius_min = 1.0D-15 / &CELL cell_dynamics = 'bfgs' press = 0 press_conv_thr = 0.01 cell_dofree = 'all' / CELL_PARAMETERS {alat} 6.210735282 -0.000000003 -0.228119407 -0.000000002 5.833791719 -0.000000015 -2.783178192 -0.000000013 5.154174308 ATOMIC_SPECIES P 30.9737 P.pz-hgh.UPF Ni 58.6934 Ni.pz-hgh.UPF ATOMIC_POSITIONS {crystal} P 0.2206418181 0.1125023368 0.3445362239 P 0.7793582199 0.8874976832 0.6554637631 P 0.7793582103 0.1125022767 0.1554637796 P 0.2206418107 0.8874977433 0.8445362534 P 0.7206419422 0.6125043167 0.3445270549 P 0.2793580398 0.3874957243 0.6554729321 P 0.2793580310 0.6125043756 0.1554729615 P 0.7206419340 0.3874956654 0.8445270715 Ni 0.2499621059 0.2500045449 -0.0000154609 Ni 0.7500379121 0.7499954551 0.0000154609 Ni 0.7500378821 0.2500044729 0.5000154429 Ni 0.2499621649 0.7499955271 0.4999845671 K_POINTS {automatic} !50 ! if molecular {gamma} 8 8 8 0 0 0 ---------------end of vc-relax.in--------------------- And the scf input is as follows, --------start of scf.in---------- &CONTROL calculation = 'scf' !verbosity = 'high' restart_mode = 'from_scratch' wf_collect = .true. nstep = 200 tstress = .true. tprnfor = .true. outdir = './' prefix = 'NiP2-monoclinic' etot_conv_thr = 1.0D-6 forc_conv_thr = 1.0D-5 pseudo_dir = '../../pp/' !tefield = .true. !add saw-like potential !dipfield = .true. !lelfield = .true. !nberrycyc = 5 !gdir = 3 !nppstr = 1 / &SYSTEM ibrav = 0 celldm(1) = 1.88964475 !celldm(2) = !celldm(3) = 9.5983431328106 nat = 12 ntyp = 2 !nbnd = !tot_charge = !tot_magnetization = !starting_magnetization(1) = !angle1(1) = !angle2(1) = ecutwfc = 120 ecutrho = 480 !if ncpp,stick to the 4* relation !nr1 = !nr2 = !nr3 = !nosym = .true. !noinv = .true. !no_t_rev = .true. ! disable the usage of magnetic symmetry operations !occupations = 'fixed' ! set to 'tetrahedra' if calculate dos occupations = 'smearing' smearing = 'gaussian' degauss = 0.01 ! check the smearing contribution to total energy and if it ! is large then try to lower the value nspin = 1 ! 1:non-polarized 2: magnetization along z axis !noncolin = .true. ! magnetization in generic direction, !lspinorb = .true. ! soc calculation use a pseudopotential with spin-orbit. !assume_isolated= '2D' !input_dft = 'vdW-DF' ! defining the DFT functional !nqx1 = 1 ! proportional to nk1; for hybrid functions !nqx2 = 1 ! proportional to nk2 !nqx3 = 1 ! proportional to nk3 !lda_plus_u = .true. !Hubbard_U(1) = 0 !Hubbard_U(2) = 0 !vdw_corr = 'DFT-D' ! Dispersion correction in vdw calculations !edir = 3 ! This is the direction of applied field !emaxpos = 0.95 !eopreg = 0.1 !eamp = 0.019446905 ! Amplitude of e-field 1a.u. = 51.4220632*10^10 V/m / &ELECTRONS electron_maxstep = 1000 conv_thr = 1.0D-10 mixing_mode = 'plain' !mixing_mode = 'local-TF' mixing_beta = 0.5 diagonalization = 'david' !diago_thr_init = 1.0D-13 ! for non-scf calculations !diago_full_acc = .true. !efield = 0.027502070 ! 1 a.u. = 36.3609*10^10 V/m !efield_cart(1) = 0.0 !efield_cart(2) = 0.0 !efield_cart(3) = 0.027502070 !startingpot = 'file' !start from existing charge file !startingwfc = 'file' / CELL_PARAMETERS {alat} 6.2577067603003895 0.0000000000000000 -0.0777025195301591 0.0000000000000000 5.5251065872474996 0.0000000000000000 -2.6753479058107592 0.0000000000000000 4.8473353267277162 ATOMIC_SPECIES P 30.9737 P.pz-hgh.UPF Ni 58.6934 Ni.pz-hgh.UPF ATOMIC_POSITIONS (crystal) P 0.2015645274872214 0.1129918772500652 0.3353648698544600 P 0.7984355105127818 0.8870081427499362 0.6646351171455352 P 0.7984354965127807 0.1129918772500652 0.1646351351455369 P 0.2015645244872176 0.8870081427499362 0.8353648978544624 P 0.7015644904872218 0.6129918982500636 0.3353648698544600 P 0.2984354915127767 0.3870081427499364 0.6646351171455352 P 0.2984354775127825 0.6129918982500636 0.1646351351455369 P 0.7015644874872180 0.3870081427499364 0.8353648978544624 Ni 0.2500000000000000 0.2500000000000000 -0.0000000000000000 Ni 0.7500000180000015 0.7500000000000000 -0.0000000000000000 Ni 0.7500000420000035 0.2500000000000000 0.5000000049999969 Ni 0.2500000049999969 0.7500000000000000 0.5000000049999969 K_POINTS {automatic} !50 ! if molecular {gamma} 8 8 8 0 0 0 --------end of scf.in------------
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