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

[Stefano de Gironcoli]

Dear Gisela Bocan,

    the calculated parallel forces are very small. I don't expect any 
significant in-plane relaxation.    perpendicular forces are significant 
and you need to relax the system.
    you should check converge w.r.t. number of surface layers and vacuum 
size.
    I would do this test in the 1x1 surface rather than in the 2x2
.
    I think your smearing parameter is also very small: 2mRy=27meV which 
would be room temperature for Fermi-Dirac, with "mv" you should be able 
to use significantly larger smearing with little or no effect.  Such a 
small smearing forces you to use a very dense k-point sampling, and it 
might not be enough. This may be the origin of erratic behaviour.
    I would try some degauss in the 0.01 - 0.03 range and test 
convergence w.r.t. k-points In the bulk... and then take equivalent (ore 
close to equivalent) sets  in the surface case

stefano
-
stefano de Gironcoli - SISSA and DEMOCRITOS

On 06/11/2011 04:35 AM, Gisela Bocan 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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