[Pw_forum] vc-relax convergence

[stefano de gironcoli]

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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