[Pw_forum] surface relaxation and non-zero parallel forces.

Gisela Bocan gbocan at gmail.com
Sat Jun 11 20:54:20 CEST 2011


Hello,
         Thanks for the reply. I did not make myself clear enough in the
previous post. Sorry for that. My final goal is to relax the surface but I
posted the file.in for a test 'scf' calculation to show the initial forces I
get. The relaxation obtained with the forces posted will not be just normal
to the surface. Even if I forced it to be, atoms within the same layer will
not relax equally. Making some further tests I managed to improve the
results by messing with mixing_beta (=0.1) and mixing_mode (='local-TF') but
the problem persists none the less for some values of the parameters (as the
one posted). I did reduce the density of k-points in later tests. In any
case, I think I finally succeded in solving the problem, at least for the
set of parameters I am interested in.
          Again thanks for the quick reply!

Dr. G. A. Bocan.
Surface Physics Group,
Centro Atómico Bariloche.


On Sat, Jun 11, 2011 at 1:48 AM, mohnish pandey <mohnish.iitk at gmail.com>wrote:

> Dear Dr. Bocan,
>                           AS your input file suggest, you are not relaxing
> the system, you are doing the calculation for only one electronic cycle. As
> you have cleaved the bulk the atoms will relax upon cleaving and they wont
> be in the bulk-like position, so you must relax all of them. Put the flag
> "calculation" = "relax", it will minimize the forces and finally you will
> get the systems with atoms having force below the threshold value(you can
> use the default value in the code). And I think there is not need to use
> such a dense grid of k- points, I think 12x10x1 will suffice.
>
> On Sat, Jun 11, 2011 at 8:05 AM, Gisela Bocan <gbocan at gmail.com> wrote:
>
>> Hello,
>>          I assumed surface relaxation for the Ag(110) surface would be an
>> easy matter. As it happens, I find non-zero forces parallel to the surface
>> and therefore, upon relaxation, I get a non-neglectable parallel
>> displacement of the ions. I am working with a 2x2sqrt(2) unit cell. I attach
>> my file.in and the forces I get. Note that the forces along one direction
>> (x) are much larger that along the other one (y). Note as well the large
>> values considered for ecutwfc, ecutrho, kpoints and the small degauss and
>> conv_thr. Any ideas on how to correct this behaviour? Am I missing
>> something? Thanks in advance and sorry if this is a stupid question. Small
>> changes in the input parameters can lower the forces I get but I find
>> nothing systematic.
>>
>> &control
>>    prefix        = 't_G0',
>>    outdir        = ' ',
>>    pseudo_dir    = '/home/qe/pseudo',
>>    calculation   = 'scf',
>>    tprnfor       = .true.,
>>    lkpoint_dir   = .false.,
>>    verbosity     = 'high',
>> /
>> &system
>>    ibrav       = 8,
>>    celldm(1)   = 11.1298607358762567,
>>    celldm(2)   = 1.4142135623730949,
>>    celldm(3)   = 2.25,
>>    nat         = 16,
>>    ntyp        = 1,
>>    ecutwfc     = 45,
>>    ecutrho     = 450.00,
>>    occupations = 'smearing',
>>    smearing    = 'mv',
>>    degauss     = 0.002,
>> /
>> &electrons
>>    conv_thr = 1.d-12
>>    diagonalization = 'cg'
>> /
>> &ions
>> /
>> ATOMIC_SPECIES
>>    Ag   107.8682  Ag.pbe-d-rrkjus.UPF
>> ATOMIC_POSITIONS  crystal
>>    Ag   0.0000000000000000   0.0000000000000000   0.3333333333333333
>>    Ag   0.5000000000000000   0.0000000000000000   0.3333333333333333
>>    Ag   0.0000000000000000   0.5000000000000000   0.3333333333333333
>>    Ag   0.5000000000000000   0.5000000000000000   0.3333333333333333
>>    Ag   0.2500000000000000   0.2500000000000000   0.2222222222222222
>>    Ag   0.7500000000000000   0.2500000000000000   0.2222222222222222
>>    Ag   0.2500000000000000   0.7500000000000000   0.2222222222222222
>>    Ag   0.7500000000000000   0.7500000000000000   0.2222222222222222
>>    Ag   0.0000000000000000   0.0000000000000000   0.1111111111111111
>>    Ag   0.5000000000000000   0.0000000000000000   0.1111111111111111
>>    Ag   0.0000000000000000   0.5000000000000000   0.1111111111111111
>>    Ag   0.5000000000000000   0.5000000000000000   0.1111111111111111
>>    Ag   0.2500000000000000   0.2500000000000000   0.0000000000000000
>>    Ag   0.7500000000000000   0.2500000000000000   0.0000000000000000
>>    Ag   0.2500000000000000   0.7500000000000000   0.0000000000000000
>>    Ag   0.7500000000000000   0.7500000000000000   0.0000000000000000
>> K_POINTS automatic
>>    33 23 1  0 0 0
>>
>>      atom   1 type  1   force =     0.00000000    0.00000000   -0.01175276
>>      atom   2 type  1   force =     0.00000000    0.00000000   -0.01175282
>>      atom   3 type  1   force =     0.00000000    0.00000000   -0.01175318
>>      atom   4 type  1   force =     0.00000000    0.00000000   -0.01175325
>>      atom   5 type  1   force =    -0.00004221    0.00000033    0.01048323
>>      atom   6 type  1   force =     0.00004221    0.00000033    0.01048323
>>      atom   7 type  1   force =    -0.00004221   -0.00000033    0.01048323
>>      atom   8 type  1   force =     0.00004221   -0.00000033    0.01048323
>>      atom   9 type  1   force =     0.00000000    0.00000000   -0.01048309
>>      atom  10 type  1   force =     0.00000000    0.00000000   -0.01048306
>>      atom  11 type  1   force =     0.00000000    0.00000000   -0.01048272
>>      atom  12 type  1   force =     0.00000000    0.00000000   -0.01048269
>>      atom  13 type  1   force =    -0.00004148    0.00000003    0.01175266
>>      atom  14 type  1   force =     0.00004148    0.00000003    0.01175266
>>      atom  15 type  1   force =    -0.00004148   -0.00000003    0.01175266
>>      atom  16 type  1   force =     0.00004148   -0.00000003    0.01175266
>>
>> Dr. G. A. Bocan
>> Surface Physics Group,
>> Centro Atómico Bariloche,
>>
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>>
>
>
> --
> Regards,
> MOHNISH,
> -----------------------------------------------------------------
> Mohnish Pandey
> Senior Project Associate,
> Department of Chemical Engineering,
> IIT KANPUR, UP, INDIA
> -----------------------------------------------------------------
>
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