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Hi Ted,</div>
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<br>
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One possibility is that a fixed set of G-vector is selected using the initial cell based on the specified `<span style="font-family:SimSun">ecutwfc</span>`. 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:
<a href="https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm406" id="LPNoLPOWALinkPreview_1">
https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm406</a><br>
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Hope that helps.</div>
<div style="font-family: "Times New Roman", Times, serif; font-size: 12pt; color: rgb(0, 0, 0);">
Hsin-Yu<br>
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<div><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Hsin-Yu Ko</span></span></div>
<div><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Postdoctoral Research Fellow<br>
</span></span></div>
<span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Department of Chemistry and Chemical Biology<br>
</span></span><span><span style="font-family:georgia,serif"><span style="color:rgb(102,102,102)">Cornell University</span></span></span></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of ÑîÌÚ <yangteng@imr.ac.cn><br>
<b>Sent:</b> Sunday, June 19, 2022 1:51 AM<br>
<b>To:</b> users@lists.quantum-espresso.org <users@lists.quantum-espresso.org><br>
<b>Subject:</b> [QE-users] Stress values from vc-relax and scf are different, but why?</font>
<div> </div>
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<div>
<p style="font-family:SimSun">Dear QE users and experts, </p>
<p style="font-family:SimSun"><br>
</p>
<p style="font-family:SimSun">I am pretty confused by the different outputed stress value from both
</p>
<p style="font-family:SimSun">the vc-relax (the last step) and scf steps. Could you please help me
</p>
<p style="font-family:SimSun">to figure out why. Thank you so much! </p>
<p style="font-family:SimSun"><br>
</p>
<p style="font-family:SimSun">Ted </p>
<p style="font-family:SimSun"><br>
</p>
<p style="font-family:SimSun"><br>
</p>
<p style="font-family:SimSun">Here is the output stress from the last step of vc-relax:
</p>
<p style="font-family:SimSun"> total stress (Ry/bohr**3) (kbar) P= 0.01<br>
0.00000028 -0.00000000 -0.00000013 0.04 -0.00 -0.02<br>
-0.00000000 -0.00000011 -0.00000000 -0.00 -0.02 -0.00<br>
-0.00000013 -0.00000000 0.00000011 -0.02 -0.00 0.02<br>
</p>
<p><br>
</p>
<p>and the output stress from scf: </p>
<p></p>
<div> total stress (Ry/bohr**3) (kbar) P= -417.74 </div>
<p> -0.00286693 0.00000000 -0.00006361 -421.74 0.00 -9.36<br>
0.00000000 -0.00279769 0.00000000 0.00 -411.55 0.00<br>
-0.00006361 0.00000000 -0.00285453 -9.36 0.00 -419.92<br>
</p>
<div><br>
</div>
<p><br>
</p>
<p>The input files for vc-relax is as below: </p>
<p></p>
<p style="font-family:SimSun; white-space:normal">---------------start of vc-relax.in---------------------
</p>
<p></p>
<p><span style="font-family:SimSun">&CONTROL</span><br>
<span style="font-family:SimSun">calculation = 'vc-relax'</span><br>
<span style="font-family:SimSun">verbosity = 'high'</span><br>
<span style="font-family:SimSun">restart_mode = 'from_scratch'</span><br>
<span style="font-family:SimSun">wf_collect = .true.</span><br>
<span style="font-family:SimSun">nstep = 200</span><br>
<span style="font-family:SimSun">tstress = .true.</span><br>
<span style="font-family:SimSun">tprnfor = .true.</span><br>
<span style="font-family:SimSun">outdir = './'</span><br>
<span style="font-family:SimSun">prefix = 'NiP2-monoclinic'</span><br>
<span style="font-family:SimSun">etot_conv_thr = 1.0D-6</span><br>
<span style="font-family:SimSun">forc_conv_thr = 1.0D-5</span><br>
<span style="font-family:SimSun">pseudo_dir = '../../pp/'</span><br>
<span style="font-family:SimSun">!tefield = .true. !add saw-like potential</span><br>
<span style="font-family:SimSun">!dipfield = .true.</span><br>
<span style="font-family:SimSun">!lelfield = .true.</span><br>
<span style="font-family:SimSun">!nberrycyc = 5</span><br>
<span style="font-family:SimSun">!gdir = 3</span><br>
<span style="font-family:SimSun">!nppstr = 1</span><br>
<span style="font-family:SimSun">/</span><br>
<span style="font-family:SimSun">&SYSTEM</span><br>
<span style="font-family:SimSun">ibrav = 0</span><br>
<span style="font-family:SimSun">celldm(1) = 1.6896</span><br>
<span style="font-family:SimSun">!celldm(2) =</span><br>
<span style="font-family:SimSun">!celldm(3) = 9.5983431328106</span><br>
<span style="font-family:SimSun">nat = 12</span><br>
<span style="font-family:SimSun">ntyp = 2</span><br>
<span style="font-family:SimSun">!nbnd =</span><br>
<span style="font-family:SimSun">!tot_charge =</span><br>
<span style="font-family:SimSun">!tot_magnetization = </span><br>
<span style="font-family:SimSun">!starting_magnetization(1) = </span><br>
<span style="font-family:SimSun">!angle1(1) = </span><br>
<span style="font-family:SimSun">!angle2(1) =</span><br>
<span style="font-family:SimSun">ecutwfc = 120</span><br>
<span style="font-family:SimSun">ecutrho = 480 !if ncpp,stick to the 4* relation </span><br>
<span style="font-family:SimSun">!nr1 =</span><br>
<span style="font-family:SimSun">!nr2 =</span><br>
<span style="font-family:SimSun">!nr3 =</span><br>
<span style="font-family:SimSun">!nosym = .true.</span><br>
<span style="font-family:SimSun">!noinv = .true.</span><br>
<span style="font-family:SimSun">!no_t_rev = .true. ! disable the usage of magnetic symmetry operations</span><br>
<span style="font-family:SimSun">!occupations = 'fixed' ! set to 'tetrahedra' if calculate dos</span><br>
<span style="font-family:SimSun"> occupations = 'smearing'</span><br>
<span style="font-family:SimSun"> smearing = 'gaussian'</span><br>
<span style="font-family:SimSun">degauss = 0.01 ! check the smearing contribution to total energy and if it </span><br>
<span style="font-family:SimSun"> ! is large then try to lower the value</span><br>
<span style="font-family:SimSun">nspin = 1 ! 1:non-polarized 2: magnetization along z axis</span><br>
<span style="font-family:SimSun"><br>
</span><br>
<span style="font-family:SimSun">!noncolin = .true. ! magnetization in generic direction,</span><br>
<span style="font-family:SimSun">!lspinorb = .true. ! soc calculation use a pseudopotential with spin-orbit.</span><br>
<span style="font-family:SimSun">!assume_isolated= '2D'</span><br>
<span style="font-family:SimSun">!input_dft = 'vdW-DF' ! defining the DFT functional</span><br>
<span style="font-family:SimSun">!nqx1 = 1 ! proportional to nk1; for hybrid functions</span><br>
<span style="font-family:SimSun">!nqx2 = 1 ! proportional to nk2</span><br>
<span style="font-family:SimSun">!nqx3 = 1 ! proportional to nk3</span><br>
<span style="font-family:SimSun">!lda_plus_u = .true.</span><br>
<span style="font-family:SimSun">!Hubbard_U(1) = 0</span><br>
<span style="font-family:SimSun">!Hubbard_U(2) = 0</span><br>
<span style="font-family:SimSun">!vdw_corr = 'DFT-D' ! Dispersion correction in vdw calculations</span><br>
<span style="font-family:SimSun"><br>
</span><br>
<span style="font-family:SimSun">!edir = 3 ! This is the direction of applied field</span><br>
<span style="font-family:SimSun">!emaxpos = 0.95</span><br>
<span style="font-family:SimSun">!eopreg = 0.1</span><br>
<span style="font-family:SimSun">!eamp = 0.019446905 ! Amplitude of e-field 1a.u. = 51.4220632*10^10 V/m</span><br>
<span style="font-family:SimSun">/</span><br>
<span style="font-family:SimSun">&ELECTRONS</span><br>
<span style="font-family:SimSun">electron_maxstep = 1000</span><br>
<span style="font-family:SimSun">conv_thr = 1.0D-10</span><br>
<span style="font-family:SimSun">mixing_mode = 'plain'</span><br>
<span style="font-family:SimSun">!mixing_mode = 'local-TF'</span><br>
<span style="font-family:SimSun">mixing_beta = 0.5</span><br>
<span style="font-family:SimSun">diagonalization = 'david'</span><br>
<span style="font-family:SimSun">!diago_thr_init = 1.0D-13 ! for non-scf calculations</span><br>
<span style="font-family:SimSun">!diago_full_acc = .true.</span><br>
<span style="font-family:SimSun">!efield = 0.027502070 ! 1 a.u. = 36.3609*10^10 V/m</span><br>
<span style="font-family:SimSun">!efield_cart(1) = 0.0</span><br>
<span style="font-family:SimSun">!efield_cart(2) = 0.0 </span><br>
<span style="font-family:SimSun">!efield_cart(3) = 0.027502070</span><br>
<span style="font-family:SimSun">!startingpot = 'file' !start from existing charge file</span><br>
<span style="font-family:SimSun">!startingwfc = 'file'</span><br>
<span style="font-family:SimSun">/</span><br>
<span style="font-family:SimSun">&IONS</span><br>
<span style="font-family:SimSun">ion_dynamics = 'bfgs'</span><br>
<span style="font-family:SimSun">upscale = 1.0D3</span><br>
<span style="font-family:SimSun">trust_radius_min = 1.0D-15</span><br>
<span style="font-family:SimSun">/</span><br>
<span style="font-family:SimSun">&CELL</span><br>
<span style="font-family:SimSun">cell_dynamics = 'bfgs'</span><br>
<span style="font-family:SimSun">press = 0</span><br>
<span style="font-family:SimSun">press_conv_thr = 0.01</span><br>
<span style="font-family:SimSun">cell_dofree = 'all'</span><br>
<span style="font-family:SimSun">/</span><br>
<span style="font-family:SimSun">CELL_PARAMETERS {alat}</span><br>
<span style="font-family:SimSun"> 6.210735282 -0.000000003 -0.228119407</span><br>
<span style="font-family:SimSun"> -0.000000002 5.833791719 -0.000000015</span><br>
<span style="font-family:SimSun"> -2.783178192 -0.000000013 5.154174308</span><br>
<span style="font-family:SimSun">ATOMIC_SPECIES</span><br>
<span style="font-family:SimSun">P 30.9737 P.pz-hgh.UPF</span><br>
<span style="font-family:SimSun">Ni 58.6934 Ni.pz-hgh.UPF</span><br>
<span style="font-family:SimSun">ATOMIC_POSITIONS {crystal}</span><br>
<span style="font-family:SimSun">P 0.2206418181 0.1125023368 0.3445362239</span><br>
<span style="font-family:SimSun">P 0.7793582199 0.8874976832 0.6554637631</span><br>
<span style="font-family:SimSun">P 0.7793582103 0.1125022767 0.1554637796</span><br>
<span style="font-family:SimSun">P 0.2206418107 0.8874977433 0.8445362534</span><br>
<span style="font-family:SimSun">P 0.7206419422 0.6125043167 0.3445270549</span><br>
<span style="font-family:SimSun">P 0.2793580398 0.3874957243 0.6554729321</span><br>
<span style="font-family:SimSun">P 0.2793580310 0.6125043756 0.1554729615</span><br>
<span style="font-family:SimSun">P 0.7206419340 0.3874956654 0.8445270715</span><br>
<span style="font-family:SimSun">Ni 0.2499621059 0.2500045449 -0.0000154609</span><br>
<span style="font-family:SimSun">Ni 0.7500379121 0.7499954551 0.0000154609</span><br>
<span style="font-family:SimSun">Ni 0.7500378821 0.2500044729 0.5000154429</span><br>
<span style="font-family:SimSun">Ni 0.2499621649 0.7499955271 0.4999845671</span><br>
<span style="font-family:SimSun">K_POINTS {automatic} !50 ! if molecular {gamma}</span><br>
<span style="font-family:SimSun">8 8 8 0 0 0</span><br>
</p>
<div style="font-family:SimSun; white-space:nowrap">---------------end of vc-relax.in---------------------
</div>
<p></p>
<p></p>
<p></p>
<p></p>
<p style="font-family:SimSun">And the scf input is as follows, </p>
<p style="font-family:SimSun">--------start of scf.in---------- </p>
<p style="font-family:SimSun">&CONTROL<br>
calculation = 'scf'<br>
!verbosity = 'high'<br>
restart_mode = 'from_scratch'<br>
wf_collect = .true.<br>
nstep = 200<br>
tstress = .true.<br>
tprnfor = .true.<br>
outdir = './'<br>
prefix = 'NiP2-monoclinic'<br>
etot_conv_thr = 1.0D-6<br>
forc_conv_thr = 1.0D-5<br>
pseudo_dir = '../../pp/'<br>
!tefield = .true. !add saw-like potential<br>
!dipfield = .true.<br>
!lelfield = .true.<br>
!nberrycyc = 5<br>
!gdir = 3<br>
!nppstr = 1<br>
/<br>
&SYSTEM<br>
ibrav = 0<br>
celldm(1) = 1.88964475<br>
!celldm(2) =<br>
!celldm(3) = 9.5983431328106<br>
nat = 12<br>
ntyp = 2<br>
!nbnd =<br>
!tot_charge =<br>
!tot_magnetization = <br>
!starting_magnetization(1) = <br>
!angle1(1) = <br>
!angle2(1) =<br>
ecutwfc = 120<br>
ecutrho = 480 !if ncpp,stick to the 4* relation <br>
!nr1 =<br>
!nr2 =<br>
!nr3 =<br>
!nosym = .true.<br>
!noinv = .true.<br>
!no_t_rev = .true. ! disable the usage of magnetic symmetry operations<br>
!occupations = 'fixed' ! set to 'tetrahedra' if calculate dos<br>
occupations = 'smearing'<br>
smearing = 'gaussian'<br>
degauss = 0.01 ! check the smearing contribution to total energy and if it <br>
! is large then try to lower the value<br>
nspin = 1 ! 1:non-polarized 2: magnetization along z axis<br>
<br>
!noncolin = .true. ! magnetization in generic direction,<br>
!lspinorb = .true. ! soc calculation use a pseudopotential with spin-orbit.<br>
!assume_isolated= '2D'<br>
!input_dft = 'vdW-DF' ! defining the DFT functional<br>
!nqx1 = 1 ! proportional to nk1; for hybrid functions<br>
!nqx2 = 1 ! proportional to nk2<br>
!nqx3 = 1 ! proportional to nk3<br>
!lda_plus_u = .true.<br>
!Hubbard_U(1) = 0<br>
!Hubbard_U(2) = 0<br>
!vdw_corr = 'DFT-D' ! Dispersion correction in vdw calculations<br>
<br>
!edir = 3 ! This is the direction of applied field<br>
!emaxpos = 0.95<br>
!eopreg = 0.1<br>
!eamp = 0.019446905 ! Amplitude of e-field 1a.u. = 51.4220632*10^10 V/m<br>
/<br>
&ELECTRONS<br>
electron_maxstep = 1000<br>
conv_thr = 1.0D-10<br>
mixing_mode = 'plain'<br>
!mixing_mode = 'local-TF'<br>
mixing_beta = 0.5<br>
diagonalization = 'david'<br>
!diago_thr_init = 1.0D-13 ! for non-scf calculations<br>
!diago_full_acc = .true.<br>
!efield = 0.027502070 ! 1 a.u. = 36.3609*10^10 V/m<br>
!efield_cart(1) = 0.0<br>
!efield_cart(2) = 0.0 <br>
!efield_cart(3) = 0.027502070<br>
!startingpot = 'file' !start from existing charge file<br>
!startingwfc = 'file'<br>
/<br>
CELL_PARAMETERS {alat}<br>
6.2577067603003895 0.0000000000000000 -0.0777025195301591<br>
0.0000000000000000 5.5251065872474996 0.0000000000000000<br>
-2.6753479058107592 0.0000000000000000 4.8473353267277162<br>
ATOMIC_SPECIES<br>
P 30.9737 P.pz-hgh.UPF<br>
Ni 58.6934 Ni.pz-hgh.UPF<br>
ATOMIC_POSITIONS (crystal)<br>
P 0.2015645274872214 0.1129918772500652 0.3353648698544600<br>
P 0.7984355105127818 0.8870081427499362 0.6646351171455352<br>
P 0.7984354965127807 0.1129918772500652 0.1646351351455369<br>
P 0.2015645244872176 0.8870081427499362 0.8353648978544624<br>
P 0.7015644904872218 0.6129918982500636 0.3353648698544600<br>
P 0.2984354915127767 0.3870081427499364 0.6646351171455352<br>
P 0.2984354775127825 0.6129918982500636 0.1646351351455369<br>
P 0.7015644874872180 0.3870081427499364 0.8353648978544624<br>
Ni 0.2500000000000000 0.2500000000000000 -0.0000000000000000<br>
Ni 0.7500000180000015 0.7500000000000000 -0.0000000000000000<br>
Ni 0.7500000420000035 0.2500000000000000 0.5000000049999969<br>
Ni 0.2500000049999969 0.7500000000000000 0.5000000049999969<br>
K_POINTS {automatic} !50 ! if molecular {gamma}<br>
8 8 8 0 0 0<span style="white-space:nowrap"></span> </p>
<p style="font-family:SimSun">--------end of scf.in------------ </p>
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