[QE-users] Large estimated SCF accuracy for monolayer \alpha-RuCl3

[Lingzhi ZHANG]

Dear QE community, 

I meet a strange SCF convergence problem for my monolayer \alpha-RuCl3 system.

I’m conducting GGA+SOC+U+Constrained DFT in QE-7.0.
But the estimated SCF accuracy for my system is considerably large in the first and the following steps, more enormous than 10000 Ry. 

I have tried many methods such as modifying the mixing scheme from plain to TF, tuning the degauss, and trying different occupation and smearing schemes.
I also tried to start from the GGA+LSDA+U case, which works well and gets converged in 700 steps. 
But no scheme works for  GGA+SOC+U+Constrained DFT.
And even for just the GGA+SOC+U case, I can't obtain a small SCF accuracy.

I’d like to know what I can do for it and get a normal SCF process. 

My input is in the following. 
This structure was obtained from OpenMX, another DFT package based on the LCAO method.

&CONTROL
    calculation='scf', 
    prefix='mono', 
    pseudo_dir='/home/zhang/calculation/rucl3/pseudo', 
    restart_mode='from_scratch',
    outdir='./', 
    verbosity='high',
/
&SYSTEM
    ibrav= 0,
    nat= 16, ntyp= 3,
    occupations = 'smearing',
    smearing='mp', degauss=1d-9,
    ecutwfc= 100,
    noncolin=.true.,lspinorb=.true.,
    starting_magnetization(1)= 0.5,
    starting_magnetization(2)= 0.5,
    starting_magnetization(3)= 0.0,
    lda_plus_u = .true.,
    lda_plus_u_kind = 1,
    U_projection_type = 'ortho-atomic',
    Hubbard_U(1)=1.5,
    Hubbard_U(2)=1.5
/
&ELECTRONS
    electron_maxstep = 1600
    conv_thr = 1.0d-8
    mixing_mode = 'TF'
    mixing_beta = 4.0d-2
    mixing_ndim = 20
    startingpot = 'atomic'
    diagonalization = 'david'

/
ATOMIC_SPECIES
Ruu     101.07       Ru.rel-pbe-spn-kjpaw_psl.1.0.0.UPF
Rud     101.07       Ru.rel-pbe-spn-kjpaw_psl.1.0.0.UPF
Cl      35.45        Cl.pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {angstrom}
    Ruu     -2.4165100      0.0119000      2.4251100
    Ruu      0.0032200      0.0119000      0.0053700
    Cl       2.3833500     -0.0499700      0.0334100
    Cl      -2.3884800     -0.0499700      4.8052300
    Cl      -0.0381100      0.0580100      2.3837700
    Cl      -2.3751800     -0.0342100      0.0467100
    Cl      -2.4518700     -2.3681500      2.5014800
    Cl       0.0796000     -2.3681500     -0.0299800
    Rud      0.0105400      2.4538100     -2.4511300
    Rud      2.4302700      2.4538100     -4.8708700
    Cl       4.8104000      2.3919400     -4.8428300
    Cl       0.0385700      2.3919400     -0.0710100
    Cl       2.3889400      2.4999200     -2.4924700
    Cl       0.0518700      2.4077000     -4.8295300
    Cl      -0.0248200      0.0737600     -2.3747600
    Cl       2.5066500      0.0737600     -4.9062200
CELL_PARAMETERS {angstrom}
     4.8541000     4.8838200    -9.7524800
    -4.8762300     2.4419100     2.4270500
    10.0203300    10.0185400    10.0203300
K_POINTS {automatic}
 6 12 1 0 0 0



The following is part of the output
total energy = -2977.28363823 Ry estimated scf accuracy < 529768.11044287 Ry
total energy = -2977.28363823 Ry estimated scf accuracy < 529768.11044287 Ry
total energy = -3184.06809008 Ry estimated scf accuracy < 814811.34802588 Ry
total energy = -3274.94372181 Ry estimated scf accuracy < 746852.52429958 Ry
total energy = -3307.21972156 Ry estimated scf accuracy < 752237.62011255 Ry
total energy = -3291.26999775 Ry estimated scf accuracy < 757458.22549562 Ry
total energy = -3349.60629608 Ry estimated scf accuracy < 765501.04417510 Ry
total energy = -3337.49045478 Ry estimated scf accuracy < 768896.10678680 Ry
total energy = -3287.13640144 Ry estimated scf accuracy < 762797.72531670 Ry
total energy = -3333.32846680 Ry estimated scf accuracy < 751861.05720886 Ry
total energy = -3341.65785225 Ry estimated scf accuracy < 754604.69881806 Ry
total energy = -3350.72599779 Ry estimated scf accuracy < 749241.08182438 Ry
total energy = -3366.01560869 Ry estimated scf accuracy < 741018.08470080 Ry



With best regards

Lingzhi Zhang
Department of Applied Physics, 
University of Tokyo, Japan