[Pw_forum] saw-like potential

[Farzad Shirazian]

Dear all,
I'm trying to study interaction of  a Sulfur atom with a Gold surface under
different electric fields and calculate its binding energy. I'm using
saw-like potentials for adding electric field. I have two questions:
1- Potential parameters : As I read the manual and pw-forum, saw like
potentials should not change slope where atoms exist but with this
condition, required parameters would not be unique and one can assign
different values to emaxpos and eopreg. What would be the difference if I
choose ,for example, *emaxpos=0.45 & eopreg=0.1* or *emaxpos=0.49 &
eopreg=0.02*? What  happens if I have a compact unit cell and the place
where slope changes is near atoms?

2- Larger electric fields: When I do my calculations for larger electric
fields, it needs more scf steps to converge as for some cases I need to
change "electron_maxstep" to a higher value than 100. Is there a trick to
speed up convergency?

Any help would be greatly appreciated.


below is a sample of the input files which I use:

 &CONTROL
                 calculation = 'scf' ,
             restart_mode = 'from_scratch' ,
                         outdir = '/home/farzad/tmp/' ,
                pseudo_dir =
'/home/farzad/Desktop/espresso-5.0.1-GPU/pseudo/' ,
                       tstress = .false. ,
                       tprnfor = .true. ,
                        tefield = .true. ,
 /
 &SYSTEM
                                  ibrav = 14,
                           celldm(1) = 15.6688,
                           celldm(2) = 1,
                           celldm(3) = 1.5,
                           celldm(4) = 0,
                           celldm(5) = 0,
                           celldm(6) = 0,
                                     nat = 9,
                                   ntyp = 2,
                              ecutwfc = 40 ,
                              ecutrho = 320 ,
                       occupations = 'smearing' ,
                             degauss = 0.01 ,
                            smearing = 'marzari-vanderbilt' ,
                                  nspin = 2 ,
   starting_magnetization(1) = 0.02,
   starting_magnetization(2) = 0.01,
                 exxdiv_treatment = 'gygi-baldereschi' ,
                                      edir = 3 ,
                              emaxpos = 0.55 ,
                                 eopreg = 0.1 ,
                                   eamp = 0.04 ,
 /
 &ELECTRONS
                electron_maxstep = 500 ,
                    diagonalization = 'cg' ,
 /
ATOMIC_SPECIES
   Au  196.96657  Au.pw91-van_ak.UPF
    S   32.06500  S.pw91-van_ak.UPF
ATOMIC_POSITIONS bohr
   Au      0.000000000    0.000000000    0.000000000
   Au      3.917200000    3.917200000    0.000000000
   Au      7.834400000    0.000000000    0.000000000
   Au     11.751600000    3.917200000    0.000000000
   Au      0.000000000    7.834400000    0.000000000
   Au      7.834400000    7.834400000    0.000000000
   Au      3.917200000   11.751600000    0.000000000
   Au     11.751600000   11.751600000    0.000000000
    S      7.834400000    3.917200000    2.050000000
K_POINTS automatic
  4 4 4   0 0 0


Best Regards,
----------------------------
Farzad Shirazian
Sharif University of Technology
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