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10

1. Is it possible that the energy gap of the material is direct in the bulk but indirect in the slab? Yes, this is possible (as is the reverse). The 2D slab does not have the same symmetry as the full 3D slab, and there is no requirement for the conduction band minima and valence band minima to be at the same points in reciprocal-space. As you make the slab ...


8

There seems to be a function to read INCAR files in ASE however in the code it mentions that the functions "are under early constructions". You can give it a try though and it might work well enough: from ase.calculators.vasp import Vasp calc = Vasp() calc.read_incar('INCAR') Alternatively, you could move over to Pymatgen which has a more thorough ...


8

This is actually an important question regardless of the code you're using (I use SIESTA and NWChem). To obtain a good phonon spectrum, the first thing you need is a truly relaxed/optimized structure. This requirement entails several parameter optimizations (mesh cutoff, force tolerance, basis set ..). Afterwards, other requirements are also important. I ...


8

TL;DR If bonds are broken in creating the lower surface of the slab, then passivation is probably a good idea. Let's start with a little bit of context: Slab calculations in computational materials science are often (but not always) designed to study the properties of a surface of a macroscopic crystal. In these cases, an accurate model of physical reality ...


7

You should take Binh Tien's advice for the continuity part of your problem. It has also been answered somewhat here: How to ensure a smooth band structure? As for the gap issue, adding to Phil Hasnip's answer, a similar question has also had several proposals for answers here : How to generate the high symmetry paths for band structure calculations? These ...


7

The community wiki by Tyberius is a bit misleading here, you are not doing molecular dynamics calculations (IBRION=0) but likely a geometry optimization (IBRION=1,2,3). In the case of geometry optimization, the CHGCAR is not the predicted charge density, but is instead the charge density of the last completed step. For this reason, you should find no ...


6

I know this was long ago, but for those that find the same problem in the future, the sisl python package might be useful. It has a generic Grid object and it knows how to read the grids from VASP: import sisl grid = sisl.get_sile("path/to/your/CHGCAR").read_grid() Then grid is a sisl Grid object, so: grid.grid contains the numpy array of values ...


6

I think this issue is avoided in practical sense. I am not sure any of the psuedopotentials provided with VASP actually include valence electrons of the same angular momentum (so you never have 3s and 4s as valence), since these states are normally part of the core. I don't think there is any reason you cannot do this in theory, but in practice the core is ...


5

No, this isn't possible for an undoped semiconductor. The reason it's happening is because the Fermi level is not well defined for your semiconductor at finite temperature, and you're trying to read the valence band maximum (VBM) from the graph of your broadened DOS. I recommend taking the VBM directly from the band energies, instead. In a DFT program, the ...


5

I don't know any VASP-specific details, but the basic answer is that the three versions are almost identical, and solve the same equations in the same way. If we take vasp_std as the reference, then the differences are: vasp_gam If your system has time-reversal symmetry (true for most simulations), then $+k$ and $-k$ are symmetry-related and we can restrict ...


5

In the passage just above what you quoted, it says: For dynamic simulation (IBRION=0), the charge density on the file is the predicted charge density for the next step: i.e. it is compatible with the CONTCAR file, but incompatible with the last positions in the OUTCAR file. This means while the density in the CHGCAR file is a density for the position given ...


5

ASE in general picks the VASP default, by simply not including the tag if not directly asked for. This is done by setting the tag to "None" which has no meaning in VASP. Then when the INCAR is generated, the tag is simply not output, leaving VASP to fall back on its default behavior. For things such as the functional, ASE does attempt to set up ...


4

# # To run VASP this script calls $vasp_std # (or posibly $vasp_gam and/or $vasp_ncl). # These variables can be defined by sourcing vaspcmd . vaspcmd 2> /dev/null As mentioned in the comments, the answer is actually in your question. vaspcmd is just any script you provide that defines the $vasp_std, $vasp_gam and $vasp_ncl variables which give the full ...


4

Following the suggestion by Ian, let's first translate SIGMA into electronic structure language: VASP's SIGMA parameter defines the smearing width of the Fermi-Dirac [1] occupation function f(E) in electron volts. There are at least two different reasons for choosing a finite smearing: You want to simulate a finite electronic temperature. You are dealing ...


4

This seems more of a thermodynamics question than a simulations question given the comment from May 12 '21. What is set to zero by definition is the Gibbs free energy of formation at standard temperature and pressure (1 atm, 298.15 K) for pure elements in their natural state. This explains the difference between the cited values for the solid and the liquid (...


4

When dealing with monolayers, I have found that using the ISIF=4 tag is better and is less time-consuming. As mentioned in Vaspwiki, it allows the relaxation of both lattice constants and atomic coordinates with a fixed volume. However, what is important here is uncommenting the FCELL(3,3)=SAVE line in constr_cell_relax.F file and recompiling VASP. This ...


4

If by "standard", you mean an "official" VASP-supported program, then the answer is no. VASP gives you the raw text output for you to deal with on your own. That being said, the vasprun.xml file produced at the end of a run contains many of the important calculation results, so any parser than can read an XML file can also parse this data ...


4

I could be wrong, but I am fairly certain that line of the CONTCAR technically doesn't matter. If you put a O instead of H, but put the potentials in the right order, you will get the right result even though the element is wrong. For visualization, it will definitely be wrong but I don't think the calculated result is wrong. In tools such as ASE the ...


3

I think the reason is many-fold. In addition to what Tristan mentioned, there are some other possible reasons: The principle quantum number is a relatively ill-defined concept for an atom in a molecule or material. The valence orbitals of an atom usually shrink somewhat upon bonding (or expand, if the atom becomes noticeably negatively charged during ...


3

This is basic bash-script which will be quite useful, just start with scratch phonopy -d --dim 2 2 2 --pa auto -c POSCAR for m in `ls POSCAR-0*` do fol=`echo ...


3

the example doesn't generate K-points from the slab, it uses the K points extracted from the Brillouin zone of the bulk primitive cell. Not sure you're reading the figure correctly. The bottom of the figure shows the three-dimensional Brillouin zone of the primitive cell, but the top of the figure (which the G-M-X-G path refers to) shows the two-dimensional ...


2

With a large enough vacuum, you don't need a dipole correction as badly to get good results. Sometimes the correction can cause convergence issues or problems with single steps giving incorrect energies. The second calculation looks like a geometry optimization, so maybe this was done to improve stability; a dipole correction should be applied after ...


1

This is related to your other question, you are correct VASP only supports fixing atoms along the X, Y, and Z axis. You can use ASE optimizers with the FixLine or FixPlane constraints to do this. You just specify which atoms are constrained and which direction they are constrained to move in. Just be aware, ASE running VASP is slower than VASP running its ...


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