Dear Stefano,<br> 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!<br>
<br>Dr. G.A. Bocan<br>Surface Physics Group<br>Centro Atómico Bariloche<br> <br><br><div class="gmail_quote">On Sat, Jun 11, 2011 at 5:03 AM, Stefano de Gironcoli <span dir="ltr"><<a href="mailto:degironc@sissa.it">degironc@sissa.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">
Dear Gisela Bocan,<br>
<br>
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.<br>
you should check converge w.r.t. number of surface layers and
vacuum size.<br>
I would do this test in the 1x1 surface rather than in the 2x2 <br>
. <br>
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.<br>
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<br>
<br>
stefano<br>
-<br>
stefano de Gironcoli - SISSA and DEMOCRITOS<div><div></div><div class="h5"><br>
<br>
On 06/11/2011 04:35 AM, Gisela Bocan wrote:
</div></div><blockquote type="cite"><div><div></div><div class="h5">
<pre>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 <a href="http://file.in" target="_blank">file.in</a> 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,
</pre>
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