[QE-users] Relaxation: bfgs failed after 4 scf cycles and 3 bfgs steps, convergence not achieved

[aleksandr.doma at yahoo.com]

Dear QE users & devs!
We get the same error using different pseudopotentials (SSSP, LDA, PBEsol) at the final steps of the structure relaxation. I mean the message:         bfgs failed after   4 scf cycles and   3 bfgs steps, convergence not achieved     (criteria: energy <  1.0E-08 Ry, force <  1.0E-08 Ry/Bohr, cell <  1.0E-01 kbar)
     End of BFGS Geometry Optimization

Using the last geometry (lattice parameters and atomic coordinates) in the next input file, QE takes 4 steps again and stops with the same problem.

We would like to achieve "ideal relaxation" for use in elastic and phonon calculations. Highlights of the input file:
&control    ....    calculation  = 'vc-relax',
    forc_conv_thr = 1.0D-8,    etot_conv_thr = 1.0D-8,    /&SYSTEM  celldm(1) =     15.89938814  celldm(2) =      1.30410009  celldm(3) =      1.47077836  ibrav       =  8,  nat         = 104,  ntyp        = 4,  ecutrho     = 800  ecutwfc     = 100/&ELECTRONS  conv_thr    = 1.D-10, (or 1.D-11)  mixing_beta=0.5/
&IONS ion_dynamics = "bfgs"/&CELL cell_dofree = "ibrav" cell_dynamics = "bfgs" press = 0.0 press_conv_thr = 0.1
K_POINTS AUTOMATIC6 5 4 0 0 0(8 6 6 0 0 0)
Restarting the 'vc-relax' calculation several times, we got the following "total force" results:
 Total force =     0.000080     Total SCF correction =     0.000018SCF correction compared to forces is large: reduce conv_thr to get better values----------------------------------------------------------------------------------------------Total force =     0.000084     Total SCF correction =     0.000037SCF correction compared to forces is large: reduce conv_thr to get better values----------------------------------------------------------------------------------------------Total force =     0.000072     Total SCF correction =     0.000002----------------------------------------------------------------------------------------------Total force =     0.000072     Total SCF correction =     0.000003     Computing stress (Cartesian axis) and pressure
          total   stress  (Ry/bohr**3)                   (kbar)     P=        0.00   0.00000000   0.00000000   0.00000000            0.00        0.00        0.00   0.00000000   0.00000001   0.00000000            0.00        0.00        0.00   0.00000000   0.00000000   0.00000002            0.00        0.00        0.00
     Message from routine bfgs:     history already reset at previous step: exiting     Energy error            =      2.4E-09 Ry     Gradient error          =      1.3E-05 Ry/Bohr     Cell gradient error     =      3.1E-03 kbaribrav =      8 celldm(1) =     15.89938798 celldm(2) =      1.30409950 celldm(3) =      1.47077720Input lattice vectors:     0.99999903     0.00000000     0.00000000     0.00000000     1.30409824     0.00000000     0.00000000     0.00000000     1.47077577New lattice vectors in INITIAL alat:     0.99999903     0.00000000     0.00000000     0.00000000     1.30409824     0.00000000     0.00000000     0.00000000     1.47077577New lattice vectors in NEW alat (for information only):     1.00000000     0.00000000     0.00000000     0.00000000     1.30409950     0.00000000     0.00000000     0.00000000     1.47077720Discrepancy in bohr =     0.000000    0.000000    0.000000
     bfgs failed after   4 scf cycles and   3 bfgs steps, convergence not achieved     (criteria: energy <  1.0E-08 Ry, force <  1.0E-08 Ry/Bohr, cell <  1.0E-01 kbar)
     End of BFGS Geometry Optimization=======================================================
next file:
Total force =     0.000071     Total SCF correction =     0.000008SCF correction compared to forces is large: reduce conv_thr to get better values-------------------------------------------------------------------------------------------------Total force =     0.000077     Total SCF correction =     0.000009SCF correction compared to forces is large: reduce conv_thr to get better values-------------------------------------------------------------------------------------------------Total force =     0.000078     Total SCF correction =     0.000000--------------------------------------------------------------------------------------------------Total force =     0.000082     Total SCF correction =     0.000001

     Computing stress (Cartesian axis) and pressure
          total   stress  (Ry/bohr**3)                   (kbar)     P=       -0.00  -0.00000001   0.00000000   0.00000000           -0.00        0.00        0.00   0.00000000  -0.00000002   0.00000000            0.00       -0.00        0.00   0.00000000   0.00000000  -0.00000001            0.00        0.00       -0.00
     Message from routine bfgs:     history already reset at previous step: exiting     Energy error            =      1.5E-09 Ry     Gradient error          =      2.2E-05 Ry/Bohr     Cell gradient error     =      2.7E-03 kbaribrav =      8 celldm(1) =     15.89938814 celldm(2) =      1.30410009 celldm(3) =      1.47077836Input lattice vectors:     1.00000001     0.00000000     0.00000000     0.00000000     1.30410010     0.00000000     0.00000000     0.00000000     1.47077837New lattice vectors in INITIAL alat:     1.00000001     0.00000000     0.00000000     0.00000000     1.30410010     0.00000000     0.00000000     0.00000000     1.47077837New lattice vectors in NEW alat (for information only):     1.00000000     0.00000000     0.00000000     0.00000000     1.30410009     0.00000000     0.00000000     0.00000000     1.47077836Discrepancy in bohr =     0.000000    0.000000    0.000000
     bfgs failed after   4 scf cycles and   3 bfgs steps, convergence not achieved     (criteria: energy <  1.0E-08 Ry, force <  1.0E-08 Ry/Bohr, cell <  1.0E-01 kbar)
     End of BFGS Geometry Optimization==========================================================

We seem to have reached a threshold as the last calculation shows an increase in "total force" and stops with the same problem after 4 steps:
 Total force =     0.000082     Total SCF correction =     0.000008
------------------------------------------------------------------------------------Total force =     0.000088     Total SCF correction =     0.000009SCF correction compared to forces is large: reduce conv_thr to get better values---------------------------------------------------------------------------------------Total force =     0.000093     Total SCF correction =     0.000000---------------------------------------------------------------------------------------
Total force =     0.000105     Total SCF correction =     0.000001
==================================================
Questions:1) What is the acceptable threshold for further calculations of elastic properties? (for example, with the relaxed "Total force = 0.000081" we had error of 200% in elastic constants using thermo_pw for tetraborate crystals)2) Can the relaxation error be avoided to obtain a more accurate geometry?
Thanks in advance!Oleksandr
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