does the energy (enthalpy) decreases as the vc-relaxation progresses ?
The local structure around the defect may be quite different. It may just take long.


I would start by performing a fixed cell relaxation with the cell parameters fixed at the undefected equilibrium values. And only after that I would start the vc-relax calculation from the so relaxed configuration to evaluate the vacancy formation volume. The local arrangement around the defect should be already more or less ok (if the supercell is large enough).

HTH
stefano


On 06/08/2017 16:40, Manu Hegde wrote:
Hello,

I am trying to optimize anatase TiO2 supercell with 48 atoms. The supercell converges in 15 BFGS cycles for a perfect cell. But when I introduce defect (by removing one of the oxygen atom), the vc-relax is not t all converging. I am using the same parameters in both the cases. Can anyone give some feedback on this.?. Here is my input,


&control
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/work/mhegde/tmp_tiodef/',
                  pseudo_dir = '/home/mhegde/tio_project/def_tio/',
                      prefix = 'tioan',
                   verbosity = 'high',

 /
 &system
                       ibrav = 0,
                       nat = 47,
                        ntyp = 2,
                     ecutwfc =40,
                     ecutrho=400,
                     occupations='smearing',
                      degauss=0.003

 /
 &electrons
                    conv_thr = 1.D-6,
                    mixing_beta=0.1
/
&ions
                    ion_dynamics='bfgs'

/
&cell
                    cell_dynamics='bfgs'

/
ATOMIC_SPECIES
   Ti  47.867000  Ti.pw91-nsp-van.UPF
   O   15.99900   O.pw91-van_ak.UPF

ATOMIC_POSITIONS angstrom
Ti       0.00000000 2.84182500       1.20970000
Ti       1.89455000 2.84182500       3.62910000
O        0.00000000 0.94727500       1.99939216
O        0.00000000 2.84182500       4.41879216
O        1.89455000 0.94727500       2.83940784
Ti      -1.89455000 0.94727500      -3.62910000
Ti       0.00000000 0.94727500      -1.20970000
O       -1.89455000  -0.94727500      -2.83940784
O       -1.89455000 0.94727500      -0.42000784
O        0.00000000  -0.94727500      -1.99939216
O        0.00000000 0.94727500      -4.41879216
Ti       0.00000000  -0.94727500       1.20970000
Ti       1.89455000  -0.94727500       3.62910000
O        0.00000000  -2.84182500       1.99939216
O        0.00000000  -0.94727500       4.41879216
O        1.89455000  -2.84182500       2.83940784
O        1.89455000  -0.94727500       0.42000784
Ti      -1.89455000  -2.84182500      -3.62910000
Ti       0.00000000  -2.84182500      -1.20970000
O       -1.89455000 2.84182500      -2.83940784
O       -1.89455000  -2.84182500      -0.42000784
O        0.00000000 2.84182500      -1.99939216
O        0.00000000  -2.84182500      -4.41879216
Ti      -3.78910000 2.84182500       1.20970000
Ti      -1.89455000 2.84182500       3.62910000
O       -3.78910000 0.94727500       1.99939216
O       -3.78910000 2.84182500       4.41879216
O       -1.89455000 0.94727500       2.83940784
O       -1.89455000 2.84182500       0.42000784
Ti       1.89455000 0.94727500      -3.62910000
Ti      -3.78910000 0.94727500      -1.20970000
O        1.89455000  -0.94727500      -2.83940784
O        1.89455000 0.94727500      -0.42000784
O       -3.78910000  -0.94727500      -1.99939216
O       -3.78910000 0.94727500      -4.41879216
Ti      -3.78910000  -0.94727500       1.20970000
Ti      -1.89455000  -0.94727500       3.62910000
O       -3.78910000  -2.84182500       1.99939216
O       -3.78910000  -0.94727500       4.41879216
O       -1.89455000  -2.84182500       2.83940784
O       -1.89455000  -0.94727500       0.42000784
Ti       1.89455000  -2.84182500      -3.62910000
Ti      -3.78910000  -2.84182500      -1.20970000
O        1.89455000 2.84182500      -2.83940784
O        1.89455000  -2.84182500      -0.42000784
O       -3.78910000 2.84182500      -1.99939216
O       -3.78910000  -2.84182500      -4.41879216

K_POINTS {automatic}

 2 2 2   0 0 0

CELL_PARAMETERS angstrom

 7.57820000       0.00000000       0.00000000
 0.00000000       7.57820000       0.00000000
 0.00000000       0.00000000       9.67760000

Thanks,
Manu
(University of Waterloo)






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