[Pw_forum] different result from vc-relax and manually scanning alat constant for crystal structure optimization

[Nicola Marzari]

Does vc-relax change also celldm(3)?

BTW, 0.06 for degauss is a tad too large.

Nicola

Sent from a tiny keyboard...

On 18 May 2013, at 22:15, Dongsheng Zhang <zhdsheng21 at gmail.com> wrote:

> Hi,
> 
> I have used vc-relax to get the optimum lattice constant for Ti HCP crystal. The result is very close to the experiment data. However, when I tried to calc the total energy at different lattice constant to get the dependency of total energy on the lattice constant, I got the different answer to the optimum lattice constant. Could anyone tell me what's wrong in my simulations? Thanks.
> 
> Dongsheng
> 
> The input for vc-relax method:
> 
> 
>  &control
>     prefix='ti',
>     pseudo_dir = '/home1/01993/zhdsheng/espresso/pseudo/',
>     outdir='/scratch/01993/zhdsheng/Ti_K6L6.0/',
>     calculation='vc-relax',
>     restart_mode='from_scratch'
>  /
>  &system    
>     ibrav=  4, celldm(1) =6.0,celldm(3) =1.633, nat=  2, ntyp= 1,
>     occupations='smearing', smearing='marzari-vanderbilt', degauss=0.06,
>     ecutwfc =150, 
>     
>  /
>  &electrons
>  /
>  &ions    
>  /
>  &cell
>     cell_dynamics='bfgs'   
>  /
> ATOMIC_SPECIES
>  Ti  47.867  Ti.pbe-mt_fhi.UPF
> ATOMIC_POSITIONS
>    Ti  0.000000000000 0.000000000000 0.000000000000
>    Ti  0.666666666666 0.333333333333 0.50000000000
> K_POINTS automatic
>    6 6 6 1 1 1 
> 
> The optimum lattice constant from this input file is 5.576
> 
> The input for manual optimization:
> template.in:
> 
> &control
>     prefix='ti',
>     pseudo_dir = '/home1/01993/zhdsheng/espresso/pseudo/',
>     outdir='/scratch/01993/zhdsheng/Ti_try/',
>     restart_mode='from_scratch'
>  /
>  &system    
>     ibrav=  4, celldm(1) =alat,celldm(3) =1.633, nat=  2, ntyp= 1,
>     occupations='smearing', smearing='marzari-vanderbilt', degauss=0.06,
>     ecutwfc =150, 
>     
>  /
>  &electrons
>  /
>  &ions    
>  /
> ATOMIC_SPECIES
>  Ti  47.867  Ti.pbe-mt_fhi.UPF
> ATOMIC_POSITIONS
>    Ti  0.000000000000 0.000000000000 0.000000000000
>    Ti  0.666666666666 0.333333333333 0.50000000000
> K_POINTS automatic
>    6 6 6 1 1 1 
> 
> alat is the lattice variable.  The result from this input file is:
> 
> alat      etotal
> 5.0760 -13.57602170
> 5.1760 -13.61996733
> 5.2760 -13.65721285
> 5.3760 -13.68841125
> 5.4760 -13.71412487
> 5.5760 -13.73484556
> 5.6760 -13.75101618
> 5.7760 -13.76303624
> 5.8760 -13.77128114
> 5.9760 -13.77609614
> 6.0760 -13.77784664
> 6.1760 -13.77685770
> 6.2760 -13.77353840
> 6.3760 -13.76853094
> 6.4760 -13.76290015
> 6.5760 -13.75652953
> 6.6760 -13.74869787
> 6.7760 -13.74207406
> 
> 
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