[Pw_forum] problem in vc-relax

[Arles V. Gil Rebaza]

Hi Ashish Kumar, if you don't know: vc-relax means variable-cell relax. So,
you're doing this kind of calculation, where the default is relax all cell
parameters ( axis, angles, position). You can choose which parameters are
fixed or which parameters are relaxed, please read the QE manual, the
section  &CELL , the variable "cell_dofree"

Best.

PhD stud Arles V. Gil Rebaza
IFLP - Argentina

2011/10/4 Ashish Kumar <ashishespresso at gmail.com>

> 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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>


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