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

[Dongsheng Zhang]

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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