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

[mohnish pandey]

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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