Charged cells -- bad formation energies
Yurko Natanzon
I'm trying to calculate the formation energies of the charge defects
(electron or hole) in magnesium hydride MgH2 and find that they are
much higher than expected.
E.g. the formation energy of a hole is -15.49 Hartree and the
formation energy of the electron impurity is 15.62 Hartree. Although
the sign seems to be correct, the energies are the two orders of
magnitude larger than expected!
The calculations were performed using the input:
<groundstate ngridk="3 3 5" rgkmax="6" chgexs="0" xctype="GGAPerdew-
Burke-Ernzerhof">
For the hole I specfiend chgexs=-1 and for the additional electron I
specified chgexs=+1. Is it the correct way of calculating the total
energy of the charged cell or anything else should be specified?
Inspection of INFO.OUT file shows that major contribution to the total
energy of the charged cell comes from "comp. background charge" term
which is -15.53 for the hole and 15.47 for the electron. If we
substract these terms we get 0.04 Hartree for the hole and 0.15
Hartree for the electron. These values are much better and comparable
to the results given by other programs.
So, my question is what is the meaning of "comp. background charge"
term? To preserve the neutrality for DFT calculations we do need to
add a compensating background charge but I haven't found an option to
do this. It seems that in the exciting it is done automatically but is
treated incorrectly (bad sign?).
with kind regards,
Yurko
my input file I used for tests goes below (for the neutral cell):
-----------------------------
input.xml------------------------------------------------
<input xsi:noNamespaceSchemaLocation="../../source/xml/
excitinginput.xsd" xmlns:xsi="http://ww
w.w3.org/2001/XMLSchema-instance" xsltpath="../../../xml/"
scratchpath="/tmp/chm/1">
<title>MgH2-U</title>
<structure primcell="true" speciespath="../../source/species/">
<symmetries HermannMauguinSymbol="P42/mnm" HallSymbol="-P 4n 2n"
SchoenfliesSymbol="D4h^14" sp
aceGroupNumber="D4h^14">
<lattice a="8.58018" b="8.58018" c="5.71567" ab="90.0" ac="90.0"
bc="90.0"></lattice>
<WyckoffPositions>
<wspecies speciesfile="Mg.xml">
<wpos coord="0.000000000 0.000000000
0.000000000"/>
</wspecies>
<wspecies speciesfile="H.xml">
<wpos coord="0.3041000000 0.3041000000
0.000000000"/>
</wspecies>
</WyckoffPositions>
</symmetries>
<crystal>
<basevect>8.58018 0.00000 0.00000</basevect>
<basevect>0.00000 8.58018 0.00000</basevect>
<basevect>0.00000 0.00000 5.71567</basevect>
</crystal>
<species speciesfile="Mg_natanzon.xml">
<atom coord="0.0000 0.0000 0.0000"/>
<atom coord="0.5000 0.5000 0.5000"/>
</species>
<species speciesfile="H_natanzon.xml">
<atom coord="0.3041 0.3041 0.0000"/>
<atom coord="0.6959 0.6959 0.0000"/>
<atom coord="0.1959 0.8041 0.5000"/>
<atom coord="0.8041 0.1959 0.5000"/>
</species>
</structure>
<groundstate ngridk="3 3 5" rgkmax="6" chgexs="1" xctype="GGAPerdew-
Burke-Ernzerhof"></ground
state>
-------------------------------end
input.xml----------------------------------------
In Mg_natanzon.xml and H_natanzon.xml I used different muffin-tin
radii, but they were the same in all calculations.