[Pw_forum] cohesive energy calculation of Zn

[Matteo Cococcioni]

Dear Kajal,

besides the fact that the isolated Zn is not really isolated in your
calculations there are some other
few things you may want to check.

1) ecutrho is normally larger than 5*ecutwfc for US PPs (typically 8* or
more). did you check convergence of energy (at least) and maybe forces wrt
this parameter?

2) do you really need such a small smearing and such a fine k-point grid?

3) conv_thr seems a bit high to me.

hth

Matteo



On Thu, Nov 11, 2010 at 6:32 AM, kajal jindal <kajalmh18 at gmail.com> wrote:

> Hi all,
>
> I am calculating the cohesive energy of Zn and I am getting a value of
> -0.734 eV which is almost 50% of the experimental value of -1.35 eV.Can
> anybody plz suggest me what is going wrong with my calculation...I read the
> forum regarding it but still unable to resolve my problem..
> Following are the input files that i used for the calculation of total
> energy of bulk Zn atom in the hexagonal crystal structure and isolated Zn
> atom respectively..
> /* input file for bulk Zn atom in crystal */
>  &control
>     calculation='scf',
>     restart_mode='from_scratch',
>     prefix='bZn'
>     wf_collect=.true.,
>     pseudo_dir = '/home/kajaljindal/pseudo/',
>     outdir= './'
> /
>  &system
>     ibrav = 4, nat= 2, ntyp= 1,celldm(1) = 5.0338, celldm(3) = 1.85829,
>     ecutwfc = 60.0,ecutrho=300, occupations ='smearing',smearing='m-v',
>     degauss=0.001,starting_magnetization(1)=0.5,nspin=2,nosym=.true.,
> /
>  &electrons
>     mixing_beta = 0.1
>     mixing_mode = 'local-TF'
>     mixing_ndim = 10
>     conv_thr =  1.0e-5
>     diagonalization = 'david'
> /
>
> ATOMIC_SPECIES
>   Zn    65.39      Zn.pbe-van.UPF
>
> ATOMIC_POSITIONS {crystal}
> Zn  0.333333  0.666666997  0.249999990
> Zn  0.666667  0.333333003  0.750000010
>
> K_POINTS {automatic}
>   9 9 6 0 0 0
>
> /* part of the corresponding output file */
>      total energy              =    -253.16323313 Ry
>      Harris-Foulkes estimate   =    -253.16322516 Ry
>      estimated scf accuracy    <       0.00000202 Ry
>
>      The total energy is the sum of the following terms:
>
>      one-electron contribution =    -133.25510851 Ry
>      hartree contribution      =      94.29714101 Ry
>      xc contribution           =     -37.02058576 Ry
>      ewald contribution        =    -177.18467547 Ry
>      smearing contrib. (-TS)   =      -0.00000441 Ry
>
>      total magnetization       =     0.00 Bohr mag/cell
>      absolute magnetization    =     0.00 Bohr mag/cell
>
> /* input file for isolated atom */
> &control
>     calculation='scf',
>     restart_mode='from_scratch',
>     prefix='iZn'
>     wf_collect=.true.,
>     pseudo_dir = '/home/kajaljindal/pseudo/',
>     outdir= './'
> /
>  &system
>     ibrav = 4, nat= 1, ntyp= 1,celldm(1) = 5.0338, celldm(3) = 1.85829,
>     ecutwfc = 60.0,ecutrho=300, occupations ='smearing',smearing='m-v',
>     degauss=0.001,starting_magnetization(1)=0.5,nspin=2,nosym=.true.,
>  /
>  &electrons
>     mixing_beta = 0.1
>     mixing_mode = 'local-TF'
>     mixing_ndim = 10
>     conv_thr =  1.0e-5
>     diagonalization = 'david'
>  /
> ATOMIC_SPECIES
>   Zn    65.39      Zn.pbe-van.UPF
>
> ATOMIC_POSITIONS {crystal}
> Zn  0.0  0.0  0.0
>
> K_POINTS {automatic}
>   9 9 6 0 0 0
>
> /* part of the corresponding output file */
>      total energy              =    -126.55461784 Ry
>      Harris-Foulkes estimate   =    -126.55460743 Ry
>      estimated scf accuracy    <       0.00000039 Ry
>
>      The total energy is the sum of the following terms:
>
>      one-electron contribution =    -125.24932971 Ry
>      hartree contribution      =      73.37661809 Ry
>      xc contribution           =     -18.43093183 Ry
>      ewald contribution        =     -56.25098076 Ry
>      smearing contrib. (-TS)   =       0.00000637 Ry
>
>      total magnetization       =     0.00 Bohr mag/cell
>      absolute magnetization    =     0.00 Bohr mag/cell
>
> I deduced twice the total energy for the isolated Zn atom from the energy
> for the bulk Zn atom in crystal to obtain the cohesive energy...Plz guide me
> where am i going wrong....
>
> Thanks in advance..
> sincerely,
> Miss Kajal
> (UTA)(University of Delhi)
> Junior Research Fellow
>
>
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>


-- 
Matteo Cococcioni
Department of Chemical Engineering and Materials Science,
University of Minnesota
421 Washington Av. SE
Minneapolis, MN 55455
Tel. +1 612 624 9056    Fax +1 612 626 7246
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