[Pw_forum] resulting atomic forces when using the "lefield" tag

[Stefano de Gironcoli]

what are the forces when you DO NOT apply any electric field ?
stefano

S. Sanchez wrote:
> Dear all,
>
>
>
> I have a question regarding the “lefield” keyword in PW.x code. I want to
> understand why if apply a positive electric field along the x-direction
> (0.01 a. u.) of a monoclinic crystal, and then, subsequently a negative
> electric field (-0.01 a. u.) along the same x-direction, the atomic forces
> (in Ry/a.u.) change not only their sign but as well their magnitude (see
> below, specially atoms 7 to 12 along the x-coordinate). In previous
> instances, I have applied positive and negative electric fields along the
> x-direction of cubic crystalline systems, and basically, I have obtained the
> same atomic forces but with different signs. In my understanding, even, if I
> have a monoclinic crystal, changing the sign of an electric field should
> only change the sign of the atomic forces, but not their magnitude; is this
> correct?
>
>
>
> Or what could be the reason for a change in both sign and magnitude of the
> atomic forces, when one only changes the sign of the applied electric field
> in monoclinic unit cells?
>
>
>
> Thanks for your help,
>
> S. Sanchez
>
>
>
>
>
> Positive Electric Field     Forces acting on atoms (Ry/au):
>
>
>
>      atom   1 type  2   force =     0.01368508   -0.00000463   -0.01954027
>
>      atom   2 type  2   force =     0.01368346   -0.00000528    0.01953860
>
>      atom   3 type  2   force =     0.01377590   -0.00017516   -0.04026512
>
>      atom   4 type  2   force =     0.01377535   -0.00017479    0.04026785
>
>      atom   5 type  2   force =     0.01376076    0.00016477    0.04122506
>
>      atom   6 type  2   force =     0.01376443    0.00016449   -0.04122388
>
>      atom   7 type  1   force =    -0.05248836    0.00005249    0.00000005
>
>      atom   8 type  1   force =     0.00550320   -0.03398525    0.00000010
>
>      atom   9 type  1   force =     0.00562059    0.03391232    0.00000004
>
>      atom  10 type  1   force =     0.01308273    0.00010950   -0.00000080
>
>      atom  11 type  1   force =    -0.02716328    0.02288385   -0.00000081
>
>      atom  12 type  1   force =    -0.02699986   -0.02294231   -0.00000081
>
>
>
>
>
> Negative Electric Field     Forces acting on atoms (Ry/au):
>
>
>
>      atom   1 type  2   force =    -0.01378038   -0.00000119   -0.01953455
>
>      atom   2 type  2   force =    -0.01378055   -0.00000160    0.01953552
>
>      atom   3 type  2   force =    -0.01374417    0.00011193   -0.04120520
>
>      atom   4 type  2   force =    -0.01374465    0.00011173    0.04120524
>
>      atom   5 type  2   force =    -0.01375505   -0.00012449    0.04022320
>
>      atom   6 type  2   force =    -0.01375511   -0.00012497   -0.04022321
>
>      atom   7 type  1   force =    -0.02545779   -0.00007625   -0.00000018
>
>      atom   8 type  1   force =     0.03331831   -0.03352278   -0.00000019
>
>      atom   9 type  1   force =     0.03321440    0.03357830   -0.00000017
>
>      atom  10 type  1   force =     0.04024209   -0.00008227   -0.00000015
>
>      atom  11 type  1   force =     0.00070042    0.02328288   -0.00000014
>
>      atom  12 type  1   force =     0.00054247   -0.02315129   -0.00000018
>
>
>
> Here is the related Quantum Espresso input file:
>
>
>
> &CONTROL
>
>     calculation='scf'
>
>     restart_mode='from_scratch',
>
>     prefix='na2o2',
>
>     lelfield=.true.,
>
>     nberrycyc=1
>
>     pseudo_dir='$PSEUDO_DIR/',
>
>     wf_collect=.true.,
>
>     verbosity='high',
>
>     tprnfor=.true.,
>
>     outdir='./'
>
> /
>
>
>
> &SYSTEM
>
> ibrav = 0,
>
> a=1,
>
> nat  =12,
>
> ntyp =2,
>
> ecutwfc = 80,
>
> ecutrho = 320,
>
> /
>
>
>
> &ELECTRONS
>
>     diagonalization='david',
>
>     conv_thr =  1.0d-8,
>
>     mixing_beta = 0.5,
>
>     startingwfc='random',
>
> efield_cart(3)=0.0
>
> efield_cart(2)=0.0
>
> efield_cart(1)=0.01 -à Change sign accordingly between 0.01 or -0.01
>
> /
>
>
>
> CELL_PARAMETERS (alat)
>
>  6.2199997902         0.0000000000         0.0000000000
>
> -3.1099998951         5.3866778298         0.0000000000
>
>  0.0000000000         0.0000000000         4.4699997902
>
>
>
> ATOMIC_SPECIES
>
> Na 23     Na.pz-n-vbc.UPF
>
> O  16.0    O.pz-mt.UPF
>
>
>
> ATOMIC_POSITIONS (crystal)
>
> O     0.000000000         0.000000000         0.166999996
>
> O     0.000000000         0.000000000         0.833000004
>
> O     0.333333343         0.666666687         0.333000004
>
> O     0.333333343         0.666666687         0.666999996
>
> O     0.666666687         0.333333343         0.666999996
>
> O     0.666666687         0.333333343         0.333000004
>
> Na    0.723999977         0.000000000         0.500000000
>
> Na    0.000000000         0.723999977         0.500000000
>
> Na    0.276000023         0.276000023         0.500000000
>
> Na    0.365999997         0.000000000         0.000000000
>
> Na    0.000000000         0.365999997         0.000000000
>
> Na    0.634000003         0.634000003         0.000000000
>
>
>
> K_POINTS {automatic}
>
> 2 2 2 0 0 0
>
>   
> ------------------------------------------------------------------------
>
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