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

[S. Sanchez]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20100815/75260672/attachment.html>