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

[Gisela Bocan]

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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