[Pw_forum] vc-relaxation question

[Yansun Yao]

Dear pwscf users,
   I met a problem when I did vc-relaxtion. First of all vc-relaxtion ran
perfect for me- I got good enough forces, stress, and total energy. But when I
use the optimized parameters as input to do a scf run, forces,stress and total
energy changed. Stress changed a lot espercially.
   I am doing a orthorhombic stucture with the Gamma only pw.x. To double check
the results, I used both ibrav=0 and ibrav=8 describtion and those two yield
the same results.
   I attached some information below. Could anybody help me to figure out the
problem? Thank you so much!
                                               Yansun Yao
vc-relax input:
&control
    calculation='vc-relax',
    restart_mode='from_scratch'
    prefix='AlOH3'
    pseudo_dir = '/home/espresso-2.1.4/pseudo/',
    outdir='/home/tmp/'
    tstress=.ture.,
    tprnfor=.true.,
    nstep=200,
    dt = 50.0,
 /
 &system
    ibrav = 0,  celldm(1)=8.3161,
    nat=  16, ntyp= 3,
    ecutwfc = 70.0, nosym=.t.,
 /
 &electrons
    mixing_beta = 0.4
    mixing_mode = 'local-TF'
    conv_thr =  1.0d-8
 /
 &ions
    upscale=20
 /
 &CELL
 cell_dynamics='damp-pr', press=0.0010,
 /
ATOMIC_SPECIES
  Al 26.98       Al.cpi.UPF
  O  15.9994     O.cpi.UPF
  H  1.008       H.cpi.UPF
ATOMIC_POSITIONS {crystal}
Al    0.0447200  0.8555000  0.7500000   1  1  0
Al    0.5447200  0.6445000  0.2500000   1  1  0
Al    0.9552800  0.1445000  0.2500000   1  1  0
**some more here**
CELL_PARAMETERS {cubic}
   1.000000000   0.000000000   0.000000000
   0.000000000   2.141773000   0.000000000
   0.000000000   0.000000000   0.646533000
K_POINTS {gamma}
************************************************
The final estimate:
  input alat =       8.3161 (a.u.)
CELL_PARAMETERS (alat)
   0.976352770   0.000021968   0.000000000
   0.000047050   2.048611386   0.000000000
   0.000000000   0.000000000   0.729286228

ATOMIC_POSITIONS (crystal)
Al       0.079415970   0.852920557   0.750000000
Al       0.579460122   0.647129732   0.250000000
Al       0.920584030   0.147079443   0.250000000
****some more here****
*************************************************
The scf input after vc-relaxation:
&control
    calculation='scf'
    pseudo_dir = '/home/espresso-2.1.4/pseudo/',
    outdir='/home/tmp/'
    title='Test aloh3 gamma only'
    prefix='aloh3_gamma'
    tprnfor=.true.,
    tstress=.true.,
 /
 &system
    ibrav = 0,  celldm(1) = 8.3161,
    nat=  16, ntyp= 3,
    ecutwfc = 70.0,
 /
 &electrons
    mixing_beta = 0.4
    mixing_mode = 'local-TF'
    conv_thr =  1.0d-8
 /
ATOMIC_SPECIES
  Al 26.98       Al.cpi.UPF
  O  15.9994     O.cpi.UPF
  H  1.008       H.cpi.UPF
ATOMIC_POSITIONS (crystal)
Al       0.079415970   0.852920557   0.750000000
Al       0.579460122   0.647129732   0.250000000
Al       0.920584030   0.147079443   0.250000000
****some more here******
CELL_PARAMETERS {cubic}
   0.976352770   0.000021968   0.000000000
   0.000047050   2.048611386   0.000000000
   0.000000000   0.000000000   0.729286228
K_POINTS {gamma}
************************************************
And the result shows:
     Total force =     0.002185     Total SCF correction =     0.000079
     entering subroutine stress ...

          total   stress  (ryd/bohr**3)                  (kbar)     P=   47.14
   0.00009568   0.00000209   0.00000000         14.08      0.31      0.00
   0.00000209   0.00003429   0.00000000          0.31      5.04      0.00
   0.00000000   0.00000000   0.00083142          0.00      0.00    122.31