[Pw_forum] about total charge and negative charge density

[Silviu Zilberman]

Hi,

I am doing an SCF calculation for a charged molecular system in vacuum. 
It is a spin unrestricted system with a total spin 1/2, and charge of 
-2. The system has 135 electrons.

When the SCF calculation starts it prints correctly

nelec=135.00

but the initial wavefunction integrates to

starting charge = 132.99857

which is as if the system is not charged. I am note sure why it happens 
and if I should worry about it.

A second problem is that I have a relatively high percentage of grid 
points with a negative charge density:

     Initial potential from superposition of free atoms

     npt with |zeta| > 1:   571576, npt tot  2985984,     19.14 %

     npt with rhoup < 0:   810487, npt tot  2985984,     27.14 %

     npt with rhodw < 0:   749915, npt tot  2985984,     25.11 %

I know that the problem is associated with representing the augmented 
charge in the ultra-soft PP, but raising the energy cutoff from 180 to 
300 Ryd seems to have no significant impact. I believe that it only 
means that the vacuum area around my system has some small charge 
oscillations that are probably not very important, but I am not 
completely sure. How does PWSCF deals with negative charge density? Is 
it set to zero or some small positive number? I couldn't find in the 
output file the value of the total negative charge density, to verify if 
it's small.

Below is my input file, and I'll greatly appreciate any comments.

Thanks, Silviu.

-------------------------------------------------------------------------------------------------------------------------------------

&control
    calculation='scf'
    pseudo_dir = '/home/silviu/Work/Pot/',
    outdir='/home/silviu/tmp/DTN_CO_hbond/'
    title='Test DTN_CO_hbond gamma only'
    prefix='DTN_CO_hbond'
    tprnfor=.true.,
    etot_conv_thr=1e-8,
    forc_conv_thr=1e-6
/
&system
    ibrav=1, celldm(1) =25.0, nat=29, ntyp= 6, nbnd=68,
    ecutwfc=30.0, nelec=135, ecutrho=300.0,
    nspin=2, nelup=68, neldw=67, occupations = 'from_input',
    starting_magnetization(1)=1.0,
    starting_magnetization(2)=0.0,
    starting_magnetization(3)=0.0,
    starting_magnetization(4)=0.0,
    starting_magnetization(5)=0.0,
    starting_magnetization(6)=0.0
/
&electrons
    diagonalization='davidson',
    mixing_mode='plain',
    mixing_beta = 0.7,
    conv_thr =  1.0d-8
/
ATOMIC_SPECIES
   Fe 55.845       fe_PBE.van
   S  31.97207070  016-S-gpbe--bm.van
   O  15.9994      008-O-gpbe--bm.van
   N  14.003074002 007-N-gpbe--bm.van
   C  12.0107      006-C-gpbe--bm.van
   H  1.0079       001-H-gpbe--bm.van
ATOMIC_POSITIONS {bohr}
Fe  0.85720064E+01   0.11652506E+02   0.12440106E+02
Fe  0.13580956E+02   0.10724365E+02   0.12776464E+02
S   0.11304998E+02   0.13262380E+02   0.15575250E+02
S   0.11532918E+02   0.13073796E+02   0.95542313E+01
S   0.17218271E+02   0.13470868E+02   0.12643983E+02
O   0.15784826E+02   0.79419992E+01   0.16960659E+02
O   0.58108997E+01   0.87424294E+01   0.86775991E+01
O   0.47305178E+01   0.15628119E+02   0.12189682E+02
O   0.10526325E+02   0.61433920E+01   0.12765676E+02
N   0.10943016E+02   0.17636183E+02   0.12431572E+02
N   0.16106110E+02   0.75557380E+01   0.86134922E+01
N   0.54538515E+01   0.92365466E+01   0.16722747E+02
C   0.12029425E+02   0.16521024E+02   0.10191009E+02
C   0.11891356E+02   0.16656483E+02   0.14795836E+02
C   0.15159513E+02   0.87937034E+01   0.10206898E+02
C   0.14947951E+02   0.90915554E+01   0.15258577E+02
C   0.69920627E+01   0.98707589E+01   0.10173031E+02
C   0.67098909E+01   0.10109927E+02   0.15104887E+02
C   0.19986851E+02   0.11413751E+02   0.12591169E+02
C   0.11092972E+02   0.83137246E+01   0.12696193E+02
C   0.63917900E+01   0.14151226E+02   0.12332573E+02
H   0.13960616E+02   0.16898237E+02   0.14860898E+02
H   0.11024086E+02   0.17719845E+02   0.16358428E+02
H   0.90163437E+01   0.17500478E+02   0.12380238E+02
H   0.14100285E+02   0.16755378E+02   0.10240702E+02
H   0.11267336E+02   0.17493751E+02   0.85188871E+01
H   0.21656485E+02   0.12570615E+02   0.12147971E+02
H   0.20295051E+02   0.10501624E+02   0.14434828E+02
H   0.19765371E+02   0.99495688E+01   0.11135089E+02
K_POINTS
1
0.0 0.0 0.0 1.0
OCCUPATIONS
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.0


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