[Pw_forum] Problem in DOS results

[Gabriele Sclauzero]

On 07/05/2010 11:54 AM, Padmaja Patnaik wrote:
> I am calculating for SiC. Below is the datas you have asked for.
>

You are lucky since I'm also studying SiC right now.

What is this supposed to be? Cubic 3C-SiC I guess? If so, I think that 
your atomic structure is wrong. There are some C atoms in the wrong 
place. If you look at it (with xcrysden, for instance), you will see 
that some tetrahedra around the carbon are upwards down. I mean, the 
should be all oriented in one direction.

Please check again your structure.

GS

>
> &system
>     ibrav=1, celldm(1) =16.4824, nat=64, ntyp=2,
>     ecutwfc = 32.0,
>     occupations='smearing', smearing='methfessel-paxton', degauss=0.02
>
>  /
> &electrons
>     mixing_beta = 0.7
>  /
> ATOMIC_SPECIES
>  Si 28.086  Si.pz-vbc.UPF
>  C  12.00   C.pz-vbc.UPF
> ATOMIC_POSITIONS
>  Si    0.000   0.000   0.000
>  Si    0.250   0.250   0.000
>  Si    0.250  -0.250   0.000
>  Si   -0.250   0.250   0.000
>  Si   -0.250  -0.250   0.000
>  Si    0.250   0.000   0.250
>  Si    0.250   0.000  -0.250
>  Si   -0.250   0.000   0.250
>  Si   -0.250   0.000  -0.250
>  Si    0.000   0.250   0.250
>  Si    0.000   0.250  -0.250
>  Si    0.000  -0.250   0.250
>  Si    0.000  -0.250  -0.250
>  Si    0.500   0.000   0.000
>  Si    0.000   0.500   0.000
>  Si    0.000   0.000   0.500
>  Si    0.500   0.250   0.250
>  Si    0.500   0.250  -0.250
>  Si    0.500  -0.250   0.250
>  Si    0.500  -0.250  -0.250
>  Si    0.250   0.500   0.250
>  Si    0.250   0.500  -0.250
>  Si   -0.250   0.500   0.250
>  Si   -0.250   0.500  -0.250
>  Si    0.250   0.250   0.500
>  Si    0.250  -0.250   0.500
>  Si   -0.250   0.250   0.500
>  Si   -0.250  -0.250   0.500
>  Si    0.500   0.500   0.000
>  Si    0.500   0.000   0.500
>  Si    0.000   0.500   0.500
>  Si    0.500   0.500   0.500
>  C     0.125   0.125   0.125
>  C    -0.125  -0.125   0.125
>  C     0.375  -0.125   0.125
>  C    -0.375   0.125   0.125
>  C    -0.125   0.375   0.125
>  C     0.125  -0.375   0.125
>  C     0.125   0.125  -0.375
>  C    -0.125  -0.125  -0.375
>  C     0.375   0.375   0.125
>  C    -0.375  -0.375   0.125
>  C     0.375  -0.125  -0.375
>  C    -0.375   0.125  -0.375
>  C    -0.125   0.375  -0.375
>  C     0.125  -0.375  -0.375
>  C     0.375   0.375  -0.375
>  C    -0.375  -0.375  -0.375
>  C    -0.125   0.125   0.125
>  C     0.375   0.125   0.125
>  C    -0.125  -0.375   0.125
>  C    -0.125   0.125  -0.375
>  C     0.375  -0.375   0.125
>  C     0.375   0.125  -0.375
>  C    -0.125  -0.375  -0.375
>  C     0.375  -0.375  -0.375
>  C     0.125  -0.125  -0.125
>  C    -0.375  -0.125  -0.125
>  C     0.125   0.375  -0.125
>  C     0.125  -0.125   0.375
>  C    -0.375   0.375  -0.125
>  C    -0.375  -0.125   0.375
>  C     0.125   0.375   0.375
>  C    -0.375   0.375   0.375
> K_POINTS {automatic}
>  2 2 2 0 0 0
>
> I have repeated runs for different smearing, different k_points also.
>  Sending you one of the input files.
>
>
>     ---------------------------------------------------------------------
>
>     Message: 1
>     Date: Mon, 5 Jul 2010 10:34:54 +0200
>     From: Gabriele Sclauzero <sclauzer at sissa.it
>     </mc/compose?to=sclauzer at sissa.it>>
>     Subject: Re: [Pw_forum] Problem in DOS results
>     To: PWSCF Forum <pw_forum at pwscf.org
>     </mc/compose?to=pw_forum at pwscf.org>>
>     Message-ID: <FC6CD37D-AD91-4AC6-81A9-F89D070980A2 at sissa.it
>     </mc/compose?to=FC6CD37D-AD91-4AC6-81A9-F89D070980A2 at sissa.it>>
>     Content-Type: text/plain; charset="iso-8859-1"
>
>     Dear Padmaja,
>
>        are you sure that your calculation has been converged wrt the
>     k-point sampling? I believe that if you build the supercell
>     correctly and change the k-point sampling accordingly you'll get
>     the same DOS. In principle this should hold even if you didn't
>     converge wrt the size of the k-point mesh, though not completely
>     sure that this is true also for tetrahedra method occupations.
>
>     Please post at least the atomic positions, lattice parameters and
>     the content of the K_POINTS input card, otherwise it will be
>     impossible to help you further...
>
>     GS
>
>     Il giorno 04/lug/2010, alle ore 21.56, Padmaja Patnaik ha scritto:
>
>     > Dear all
>     >
>     > I am studying the properties of a compound semiconductor using
>     super cell calculations. I have generated super cells by expanding
>     the lattice constant 2times, 3 times and 4 times in all the
>     directions. My calculations gives correct results for 2 times and
>     4 times extended super cells but not for 3 times extended onel. I
>     have made sure that the generated atomic co-ordinates are correct.
>     I have tried fixing all the required parameters like, k points,
>     ecutwfc and nbnd. But still its not working. after doing the scf
>     calculations I do the DOS with occupations=tetrahedra. But the DOS
>     plot for the second case is found to be much different from
>     expected while for all other cases the results match. What can be
>     the probable mistake i am doing? Please suggest.
>     >
>     > Padmaja Patnaik
>     > Research Scholar
>     > Dept of Physics
>     > IIT Bombay
>     > Mumbai, India
>


-- 

Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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