[Pw_forum] Different results

[Rafael Julian Gonzalez Hernandez]

Dear all,

I am doing a spin-polarized slab calculation for (0001)GaN 2x2 surface. I have a problem here with the total force and magnetization values.

1- First, I have performed relax calculations and I found:

a)INPUT_FILE:
 &CONTROL
                       title = 'clean (0001)GaN' ,
                 calculation = 'relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = './tmp' ,
                  pseudo_dir = '/usr/local/espresso-4.0.5/pseudo' ,
                      prefix = 'relaxation' ,
               etot_conv_thr = 1.0D-5 ,
               forc_conv_thr = 1.0D-4 ,
                       nstep = 200 ,
                     tstress = .true. ,
                     tprnfor = .true. ,
 /
 &SYSTEM
                       ibrav = 4,
                   celldm(1) = 12.162279408229,
                   celldm(3) = 3.196395659,
                         nat = 36,
                        ntyp = 3,
                     ecutwfc = 30 ,
                     ecutrho = 300 ,
                 occupations = 'smearing' ,
                     degauss = 0.02 ,
                    smearing = 'methfessel-paxton' ,
                       nspin = 2 ,
   starting_magnetization(1) = +0.7,
   starting_magnetization(2) = +0.7,
   starting_magnetization(3) = +0.1,
 /
 &ELECTRONS
            electron_maxstep = 200,
                    conv_thr = 1.0D-8 ,
                 startingpot = 'atomic' ,
                 startingwfc = 'atomic' ,
                 mixing_mode = 'plain' ,
                 mixing_beta = 0.20 ,
                 mixing_ndim = 8,
             diagonalization = 'david' ,
            diago_david_ndim = 4 ,
 /
 &IONS
                ion_dynamics = 'bfgs' ,
                 phase_space = 'full' ,
           pot_extrapolation = 'second_order' ,
           wfc_extrapolation = 'second_order' ,
                     upscale = 50.0D0 ,
            trust_radius_max = 0.5D0 ,
            trust_radius_min = 1.0D-5 ,
            trust_radius_ini = 0.4D0 ,
...

a)OUTPUT_FILE:

!    total energy              = -3168.48291997 Ry

     total magnetization       =     0.01 Bohr mag/cell
     absolute magnetization    =     0.01 Bohr mag/cell

     Total force =     0.000115     Total SCF correction =     0.000007


2- After, with the same conditions I have taken the final positions of my relax calculation and I have performed scf calculations, but I have different results:

b) calculation = 'scf'
   starting_magnetization(1) = +0.70,
   starting_magnetization(2) = +0.70,
   starting_magnetization(3) = +0.10,

I found:

!    total energy              = -3168.48291732 Ry

     total magnetization       =     0.30 Bohr mag/cell
     absolute magnetization    =     0.31 Bohr mag/cell

     Total force =     0.054697     Total SCF correction =     0.000239

c) calculation = 'scf'
   starting_magnetization(1) = +0.01,
   starting_magnetization(2) = +0.01,
   starting_magnetization(3) = +0.01,

I found:

!    total energy              = -3168.48291963 Ry

     total magnetization       =     0.11 Bohr mag/cell
     absolute magnetization    =     0.12 Bohr mag/cell

     Total force =     0.054783     Total SCF correction =     0.000141


Could anyone help me with this?

Thank you,

Rafael Julián González H.
Departamento de Fisica
Universidad Nacional de Colombia
Bogotá, Colombia.
rgonzalezh at unal.edu.co