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 RyAs 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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