[Pw_forum] force in sawlike potential

[Aihua Zhang]

Dear all,

I comment out the line to impose total force = 0 in forces.f90, and perform
a calculation of one atom in sawlike potential. The input file is attached
below. I had expected the force exerted on nucleus by the field  would
cancel the force on ion due to change in electron charge density, so the
total force should vanish since the system is neutral. I also varied the
cutoff energy, but failed to see a convergence behavior as shown below.
Could anyone give me a clue? Thank you!

Ecut(Ry)   Total force(Ry/au)
30            0.002729
40            0.003052
50            0.003848
60            0.000086
70            0.000962
80            0.000137
90            0.003208
100          0.001271

The input file:
-------------------------------------------------------------------------
 &control
    calculation = 'scf'
    restart_mode='from_scratch',
    prefix='c',
    tstress = .true.
    tprnfor = .true.
    pseudo_dir = '.',
    outdir='/scratch/local'
    tefield=.true.
 /
 &system
    ibrav=  0, nat=  1, ntyp= 1,
ecutwfc=100
    edir=3
    emaxpos=0.95
    eopreg=0.1
    eamp=0.001
 /
 &electrons
    diagonalization='cg'
    mixing_mode = 'plain'
    mixing_beta = 0.7
    conv_thr =  1.0d-8
 /
CELL_PARAMETERS
10 0 0
0 10 0
0 0 20
ATOMIC_SPECIES
 C 1 C.pbe-rrkjus.UPF
ATOMIC_POSITIONS {bohr}
 C 0.00 0.00 9.0
K_POINTS gamma
----------------------------------------------------------------------------

Best regards

zah

-- 
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Research Fellow,
Dept. of Physics, National Univ. of Singapore
10 Kent Ridge Crescent, Singapore, 119260
Tel: +65 6516 7844
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