[Pw_forum] surface relaxation and non-zero parallel forces.
Gisela Bocan
gbocan at gmail.com
Sat Jun 11 21:15:14 CEST 2011
Dear Stefano,
Thanks very much for your reply and suggestions. I will
try and check how the forces react to changing degauss (and kpoints). As I
mentioned in a previous reply, I managed to solve the problem by messing
with mixing_beta and mixing_mode. Those tests were done with kpts=10x7x1,
ecutrho=10*ecutwfc and degauss=0.002. The problem persisted for
ecutwfc=45Ry.. but I think ecutwfc=35 Ry is more than enough for me. I might
be getting too picky... Anyway.. thanks again!
Dr. G.A. Bocan
Surface Physics Group
Centro Atómico Bariloche
On Sat, Jun 11, 2011 at 5:03 AM, Stefano de Gironcoli <degironc at sissa.it>wrote:
> 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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