[QE-users] Total force not converge on gelatin

[Ying Chunhua]

Dear QE Users,

Greetings,

I have been studying QE for half a year, currently I am working on optimizing the crystal structure of gelatin molecule. The initial coordinates was obtained from a protein ‘.pdb’ file of gelatin. I do not know whether it is the best configuration so that I use QE to run the vc-relax. 

The problem I have come across is that the  ‘forc_conv_thr’ is hard to be achieved,  when I plot the history of force, I can see after a certain number of iterations, the total force would vibrate around a certain value(~1e-2) a little bit. Theoretically the total force should be decreasing to 0,  I have tried to adjust the input parameters, like ecutwfc(50~150), mixing_beta(0.1 ~ 0.7), bfgs to damp, but not effective. Please give any suggestions on what should I do? 


Thanks a lot and Best regards.
Chunhua 

The input can be seen below:

&CONTROL
    calculation   = "vc-relax"
    forc_conv_thr =  1.00000e-06
    etot_conv_thr =  1.00000e-04
    max_seconds   =  28800
    outdir='./outdir'
    pseudo_dir    = "/home/chunhua/QuantumEspresso/PAW_precision_pseudos"
    restart_mode  = "from_scratch"
    nstep=100
/

&SYSTEM
    degauss     =  1.00000e-02
    ecutrho     =  5.00000e+02
    ecutwfc     =  1.00000e+02
    ibrav       = 0
    nat         = 217
    ntyp        = 4
    occupations = "smearing"
    smearing    = "gaussian"
/

&ELECTRONS
    conv_thr         =  1.00000e-04
    electron_maxstep = 200
    mixing_beta      =  1.00000e-01
    startingpot      = "atomic"
    startingwfc      = "atomic+random"
/

&IONS
    ion_dynamics = "bfgs"
/

&CELL
    cell_dofree    = "all"
    cell_dynamics  = "bfgs"
    press_conv_thr =  5.00000e-01
/

K_POINTS {gamma}

CELL_PARAMETERS {angstrom}
32.039000   0.000000   0.000000
  0.000000  30.039000   0.000000
  0.000000   0.000000  50.039000

ATOMIC_SPECIES
O      15.99940  O.pbe-n-kjpaw_psl.1.0.0.UPF
N      14.00674  N.pbe-n-kjpaw_psl.1.0.0.UPF
H       1.00794  H.pbe-kjpaw_psl.1.0.0.UPF
C      12.01070  C.pbe-n-kjpaw_psl.1.0.0.UPF


ATOMIC_POSITIONS {angstrom}
N       7.659071   9.660491  38.488224
H       7.708071   8.734491  38.052224
C       6.804071   9.663491  39.674224
H       6.853071  10.650491  40.137224
C       7.465071   8.662491  40.595224
O       8.658071   8.812491  40.838224
C       5.336071   9.344491  39.352224
H       4.737071   9.431491  40.259224
……………………….
……………………….

-----------------------------------------------------------
Chunhua Ying
Ph.D. Student
School of Mechanical and Materials Engineering
Washington State University 
Engineering Laboratory Building, 105
900 NE College Ave, Pullman, WA 99164

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