[Pw_forum] Problem in Ionic relaxation and MD runs

[Holger Hesske]

Dear pwscf-users, -developers and other readers,

Currently I calculate BaO bulk (and surface) systems. Therefore I use a 
testsystem with a cubic unitcell containing 8 atoms. Varying the cell length 
continously (in separate scf calculations (5.00-6.00 Angstrom)) gives a 
minimum at 5.460 Angstrom, which is 1.1% less then expt. X-ray data and 
therfore acceptable (or not?). Btw: a reference stated BaO cell length with 
PBE functional to be 5.59 
One of my input files is given below: 
 
&control
    calculation='scf'
    restart_mode='from_scratch',
    pseudo_dir = '/cluster/home/chab/hesskeh/pseudo',
    outdir='/cluster/work/chab/hesskeh/BaO_bulk/CELL_ABC_USPP',
    disk_io = 'high',
 /
 &system
      ibrav = 0,
          A = 5.400,
          B = 5.400,
          C = 5.400,
          cosAB = 0, cosAC = 0, cosBC = 0,
          nat = 8,        ntyp = 2,
          ecutwfc = 50,     ecutrho = 400,
  /
 &electrons
    conv_thr =  1.0d-8,
    mixing_beta = 0.7,
    startingwfc= 'atomic+random', 
/
CELL_PARAMETERS
  1.000 0.000 0.000
  0.000 1.000 0.000
  0.000 0.000 1.000
ATOMIC_SPECIES
 Ba 137.427 Ba.pbe-nsp-van.UPF
 O   16.000 O.pbe-van_ak.UPF
ATOMIC_POSITIONS
 Ba 0.000 0.000 0.000
 Ba 0.500 0.500 0.000
 Ba 0.500 0.000 0.500
 Ba 0.000 0.500 0.500
 O  0.500 0.000 0.000
 O  0.000 0.500 0.000
 O  0.000 0.000 0.500
 O  0.500 0.500 0.500
K_POINTS automatic
 5 5 5 0 0 0 

so far so good. Moving on to ionic relaxation (nosym =.true , 
remove_rigid_rot=.true. ) :

(1) Starting from ideal atomic positions the forces where below 3.4D-6 (which 
is expected)

(2) Distortion of one O-atom (O8) from {0.5 0.5 0.5} to {0.45 0.5 0.5} led to 
force on O8 pointing to -x 0 0, so the distance between Ba and O will be 
further decrease? (No ionic movement with the default forc_conv_thr)

(3) Reducing forc_conv_thr to 1D-4 led to movement of ions but it seems not to 
converge. (I stopped the calculation after 100 steps of bfgs or damped 
dynamics and the position of the atoms seemed to be similar and not very 
different (+-0.002 crystal coordinates) from starting positions)

(4) Distortion of O8 to {0.25 0.5 0.5} or {0.75 0.5 0.5} led in the first step 
of bfgs to a force on the atom pointing to the "right" positions but from the 
second bfgs step the force on O8 is again the "wrong" direction (see below). 
The calculation did not converge and O8 move +-0.02 around its starting 
positions

BFGS (1 Step)
     atom   1 type  1   force =    -0.00099130   -0.00000711   -0.00000713
     atom   2 type  1   force =    -0.00364813    0.00000905    0.00000915
     atom   3 type  1   force =    -0.00364847    0.00000904    0.00000910
     atom   4 type  1   force =    -0.48689518   -0.00000708   -0.00000718
     atom   5 type  2   force =    -0.00003749    0.00000905    0.00000911
     atom   6 type  2   force =     0.00461601   -0.00000709   -0.00000715
     atom   7 type  2   force =     0.00461686   -0.00000710   -0.00000714
     atom   8 type  2   force =     0.48598771    0.00000122    0.00000124
     Total force =     0.687983     Total SCF correction =     0.000091
ATOMIC_POSITIONS (after 1 Step)
Ba   	 -0.000033151  -0.000000237  	-0.000000502
Ba 	  0.499743037   0.500000638 	 0.00000644
Ba      0.499743013   0.00000637   	 	 0.500000641
Ba     -0.034295409   0.499999501 		 0.499999495
O        0.499997359   0.000000638   	 0.000000642
O        0.000325137   0.499999501  	-0.000000504
O        0.000325197  -0.000000500   	 0.499999497
O        0.284231490   0.500000086  	 0.500000088
BFGS (2 Step)
     atom   1 type  1   force =     0.01415644   -0.00216324   -0.00216320
     atom   2 type  1   force =    -0.03044379    0.00262967    0.00263000
     atom   3 type  1   force =    -0.03044377    0.00263004    0.00262963
     atom   4 type  1   force =     0.14450759   -0.00226772   -0.00226767
     atom   5 type  2   force =     0.00932027    0.00263080    0.00263076
     atom   6 type  2   force =    -0.01123409   -0.00216209   -0.00216194
     atom   7 type  2   force =    -0.01123442   -0.00216198   -0.00216206
     atom   8 type  2   force =    -0.08462822    0.00086451    0.00086449
     Total force =     0.174697     Total SCF correction =     0.000033


The question arises: where is the failure or what did I wrong? Or even better:  
what should I do to get a working relaxation? 
(Caused by these confusing results I calculate scf's of the BaO cell with 
2.6-2.8 angstrom cell length and the total energy was even lower than 
the "minimum" at 5.46) 

Maybe with the answer of the question above the next part might be unimportant 
or useless. But: Starting md's from ideal atomic positions (with and without 
temp rescaling, dt ={2, 5, 10, 15, 20}) it always ended up finally with pairs 
of BaO having a interatomic distance below 1.3 Angstrom! I guess it is the 
same problem...  

Thanks for reading the complete text and in advance for every hint I get.
   
best regards

Holger Hesske 
 
PS. I could provide a lot of input and output files but i dont want to waste 
the time of the readers, in case I made a simple (stupid) mistake in setting 
up the calculations.

-- 
Dr. Holger Hesske
Inst.f.Chemie-/Bioingenieurwissenschaft.
Wolfgang-Pauli-Str. 10
ETH Hönggerberg, HCI E 133
CH-8093 Zürich
Tel: +41 44 633 66 19
Fax: +41 44 632 11 63
E-Mail: holger.hesske at chem.ethz.ch