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

S. Sanchez lcqsigi at gmail.com
Mon Aug 16 04:36:34 CEST 2010

```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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