[Pw_forum] problem in vc-relax

[Stefano de Gironcoli]

If you know where the atoms and the cell should be, then you do not need 
vc-relax (not even relax)...
but the code apparently disagrees with your thoughts.
trying to figure out why is probably going to increase your understanding.
have you visualized the initial and final configuration by XCrysDen or 
other visualization tool?
is it what you think ?

stefano

On 10/04/2011 11:43 AM, Ashish Kumar wrote:
> Hello to all,
>
> I have tried for the vc-relax of Mo2C hexagonal system, job is not done
> properly it come out in between and the atomic positions
>
> what I have given in the input file and what I am getting in the out file is
> different.
> (i) Why the crystal positions changes. what I think that the nature of the
> atomic postions must be retained.
>
> In Input file
> ATOMIC_POSITIONS (alat)
> C        0.00  0.00 0.00
> Mo       0.3333333   0.666666   0.25
> Mo       0.6666666   0.333333   0.75
>
>
>
> In out file we get
>
> CELL_PARAMETERS (alat=  5.67296009)
>     0.946518633   0.001215132  -0.004855406
>    -0.474470641   0.819090796  -0.000958606
>     0.009278926   0.007480907   1.741942673
>
> ATOMIC_POSITIONS (alat)
> C        0.156590059   0.041446237  -0.212516996
> Mo       0.157761886   0.589915381   0.269109338
> Mo       0.636664528   0.321113609   1.041609035
>
>
>
>
> (ii) why cell_parameter changes in the out file . (I think that for
> hexagonal system it must be what given in the input file)
>
>
> my input file
>
> &control
>      calculation='vc-relax'
>      restart_mode='from_scratch',
>      pseudo_dir = '/home/ashish/softwares/espresso/pseudo/',
>      outdir='/home/ashish/dft/espresso_mo2c/test1/tmp_espresso/'
>      prefix='mo2c'
>      tprnfor = .true.,
>      tstress = .true.
>   /
>   &system
>      ibrav=4, celldm(1) =5.672960087 , celldm(3)=1.573617588 , nat=3, ntyp=2,
>      nspin = 1,  starting_magnetization(1)=0.7,
>      ecutwfc = 24.0, ecutrho = 288.0,
>      occupations='smearing', smearing='methfessel-paxton', degauss=0.02
>   /
>   &electrons
>      diagonalization='cg'
>      conv_thr = 1.0e-8
>      mixing_beta = 0.7
> /
> &ions
>    ion_dynamics='bfgs'
>   /
> &cell
> cell_dynamics='bfgs'
> /
> ATOMIC_SPECIES {crystal}
>   C  12.01 C.pw91-van_ak.UPF
>   Mo 95.94 Mo.pw91-n-van.UPF
> ATOMIC_POSITIONS
>   C 0.0 0.0 0.0
>   Mo 1/3 2/3 1/4
>   Mo 2/3 1/3 3/4
>
> K_POINTS automatic
>   4 4 4   0 0 0
>
>
>   can any one help me. What is the mistake (in my input file or else)
>
>
> with best wishes
> Ashish
>
>
>
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