The "Hartree", "one-electron", "Ewald" contributions to the energy have
something in common: the G=0 contribution to the energy separately
diverges. Their sum however does not diverge for neutral cells. So one
subtracts out the diverging terms and is left with a finite G=0
contribution. This may however vary a lot between different structures,
even if the sum of the three contributions is quite the same as in this
case.
Paolo
On 25/07/2024 18:43, Lenz Fiedler wrote:
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Dear QE users,
I have a question regarding a 2D QE calculation. Consider a very simple
bilayer graphene system, which I calculate either with a "big" (23
Angstrom in z-direction) or "small" (17 Angstrom in z-direction) cell
(Input files are at the end of the message). The C atoms are in two
planes in x-y-direction.
Now both calculations are the same except for their z-dimension. In the
"small" case I make the cell a bit smaller in z-direction. I did careful
calculations to determine by how much I can make it smaller without the
total energy deviating too much from the original cell. The input I
share here is still well within the range where the total energy error
is very small. Of course, eventually one hits a point where there is a
noticeable energy difference.
Now to my question: I have noticed that for both calculations, total
energies are essentially the same, but what differs quite drastically,
are the individual energy contributions. Namely I get:
"big":
! total energy = -48.18986746 Ry
estimated scf accuracy < 0.00003985 Ry
smearing contrib. (-TS) = -0.00000000 Ry
internal energy E=F+TS = -48.18986746 Ry
The total energy is F=E-TS. E is the sum of the following terms:
one-electron contribution = -1245.11933758 Ry
hartree contribution = 624.03689026 Ry
xc contribution = -17.17420972 Ry
ewald contribution = 590.06678957 Ry
"small":
! total energy = -48.18985199 Ry
estimated scf accuracy < 0.00007424 Ry
smearing contrib. (-TS) = -0.00000000 Ry
internal energy E=F+TS = -48.18985199 Ry
The total energy is F=E-TS. E is the sum of the following terms:
one-electron contribution = -759.11502918 Ry
hartree contribution = 381.02359494 Ry
xc contribution = -17.17243795 Ry
ewald contribution = 347.07402021 Ry
As you can see, the total energies match up nicely, but all other terms
except for the XC energy are different. Is this by design? Is there a
numerical reason for it? I am trying to come up with a way to determine
the minimal vacuum in z-direction I have to leave below and above my
graphene planes, and had originally only checked the total energy. Now I
am wondering if this is enough?
Kind regards
Lenz Fiedler
PhD student (Helmholtz-Zentrum Dresden-Rossendorf)
PS: I also disabled "assume_isolated = '2D'" - I still see the same
behavior, albeit with different numbers for the energy contributions,
the total energies are still the same.
Input files:
(PSP taken from http://www.pseudo-dojo.org/index.html):
"big":
&CONTROL
calculation = 'scf'
verbosity = 'high'
restart_mode = 'from_scratch'
outdir = 'temp'
prefix = 'C'
pseudo_dir = './'
/
&SYSTEM
ibrav = 0
nbnd = 10
ecutwfc = 80
ecutrho = 320
nosym = .true.
noinv = .true.
occupations = 'smearing'
degauss = 0.0018874
smearing = 'fermi-dirac'
ntyp = 1
nat = 4
assume_isolated = '2D'
/
&ELECTRONS
conv_thr = 7.599999999999999e-05
mixing_mode = 'plain'
mixing_beta = 0.1
/
&IONS
/
&CELL
/
ATOMIC_SPECIES
C 12.011 C_ncsr_0.5_pbe.upf
K_POINTS automatic
8 8 1 0 0 0
CELL_PARAMETERS angstrom
2.4622899999999999 0.0000000000000000 0.0000000000000000
1.2310682316596653 2.1324961988004443 0.0000000000000000
0.0000000000000000 0.0000000000000000 23.3000000000000007
ATOMIC_POSITIONS angstrom
C 2.4622407526646328 0.0000426499239760 9.9535270000000011
C 1.2310701657767995 0.7108036329841640 9.9532939999999996
C 1.2310805438773487 2.1324748738384565 13.3464730000000014
C 2.4620787720005488 1.4216712408542924 13.3467060000000011
"small":
same as big but with:
CELL_PARAMETERS angstrom
2.4622899999999999 0.0000000000000000 0.0000000000000000
1.2310682316596653 2.1324961988004443 0.0000000000000000
0.0000000000000000 0.0000000000000000 17.3040246000000018
ATOMIC_POSITIONS angstrom
C 2.4622407526646328 0.0000426499239760 6.9555393000000016
C 1.2310701657767995 0.7108036329841640 6.9553063000000002
C 1.2310805438773487 2.1324748738384565 10.3484853000000019
C 2.4620787720005488 1.4216712408542924 10.3487183000000016
--
Lenz Fiedler, M. Sc.
PhD Candidate | Machine Learning for Materials Design
Tel.: +49 3581 37523 55
E-Mail: [email protected]
https://www.casus.science
CASUS - Center for Advanced Systems Understanding
Helmholtz-Zentrum Dresden-Rossendorf e.V. (HZDR)
Conrad-Schiedt-Straße 20
02826 Görlitz
Vorstand: Prof. Dr. Sebastian M. Schmidt, Dr. Diana Stiller
Vereinsregister: VR 1693 beim Amtsgericht Dresden
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--
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216
X SCUOLA ENERGIE RINNOVABILI 16-21/9/2024 - PRE-ISCRIZIONE
https://agenda.infn.it/event/39573/
_______________________________________________
The Quantum ESPRESSO community stands by the Ukrainian
people and expresses its concerns about the devastating
effects that the Russian military offensive has on their
country and on the free and peaceful scientific, cultural,
and economic cooperation amongst peoples
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
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