[QE-users] Question About "Atom Charges"

[Victor Bermudez]

Hello, 

         I hope this isn't a dumb question, but I've been all over user forum posts, tutorials and user guides and I still can't find an answer. Recent versions of QE print a table of "atom charges" upon convergence of each SCF cycle. What do these numbers mean ? I'm aware that accurate estimates of atom charges require something like a Bader analysis, but I'd still like to know how to interpret these "atom charge" values. 

         For example, here's output (input given below) for a free NO2 molecule. This is just a "simple" example of a general result. 

Magnetic moment per site:
     atom:    1    charge:    1.9288    magn:    0.1688    constr:    0.0000
     atom:    2    charge:    2.9353    magn:    0.1673    constr:    0.0000
     atom:    3    charge:    2.9339    magn:    0.1665    constr:    0.0000 

Atom 1 is N, and atoms 2 and 3 are O. The core charges (Zval) are +5 for N and +6 for O, giving a total of +17. For a charge-neutral molecule I would therefore expect the "charge" number to sum to 17 if these numbers represent electron densities. Instead the sum is 7.798 |e|. If the charges are in Rydberg atomic units (1 au = |e|/rt(2)) then the sum is actually 5.515 |e|. 

Another question I have concerns the "magnetization" values. What quantities do they represent, and what are the units ? 

         Here's the input (for a cubic lattice of molecules with a 20-Angstrom lattice constant): 
*******************************************************************************
&CONTROL
calculation='relax',
restart_mode='from_scratch',
title='free NO2; start with #237406 opt.; 200/800 cut-offs',
pseudo_dir="/p/home/bermudvm/Pseudo",
outdir="/p/work1/workspace/bermudvm/2753418.THUNDER/tmp",
! outdir='/lustre/cmf/scratch/b/bermudez/171133/',
etot_conv_thr=1.0D-5,
forc_conv_thr=1.0D-4,
nstep=500,
verbosity='default',
max_seconds=21500.0
/

&SYSTEM
ibrav=1,
a=20.0,
nat=3,
ntyp=2,
ecutwfc=200.0,
ecutrho=800.0,
occupations='smearing',
smearing='gaussian',
degauss=0.001,
nspin=2,
starting_magnetization(1)=0.20,
starting_magnetization(2)=0.0,
! nosym=.true.,
! nbnd=14,
vdw_corr='TS'
/

&ELECTRONS
electron_maxstep=100,
conv_thr=1.0D-9,
mixing_beta=0.7,
mixing_mode='plain'
! mixing_ndim=8
/

&IONS
ion_dynamics='bfgs',
upscale=10000.D0
/

ATOMIC_SPECIES
N 14.0067 N_ONCV_PBE-1.0.upf 
O 15.9994 O_ONCV_PBE-1.0.upf

ATOMIC_POSITIONS angstrom
N       -0.016847220  -0.000000025  -0.014422960
O        1.092527105   0.000000576   0.456691560
O       -1.125679885  -0.000000551   0.457731400

K_POINTS gamma

*************************************************************

Any advice would be much appreciated.

Vic Bermudez
US Naval Research Lab. (retired)