10

Broadly, there are two flavors of Density Functional Theory (DFT): Orbital Free DFT and Kohn-Sham DFT (KS-DFT). In Orbital Free DFT, we do indeed directly optimize the density until we find the minimum energy. In KS-DFT, however, we instead optimize the coefficients of the KS orbitals, which are the solutions to the KS equations: $$\left(-\frac{1}{2}\nabla^2 ...


8

This isn't possible to answer with any certainty without trying it. Strictly speaking, if you need a Hubbard U to correct for the spurious self-interaction, then you need it to model the material accurately - including modelling the forces, dynamical matrix etc. Having said that, it is usually found that the forces are very similar with and without U, at ...


8

"I will have to start looking next year into DFT techniques [...]" We have had some questions about how to get started on learning DFT here on this site before, and have received plenty of answers: I am a beginner in DFT. What are some resources that could help me to learn the basics? What are good resources to learn Materials Modeling? (this is ...


8

The Fermi energy is defined implicitly, by requiring that the total of the band occupancies $f$ is the number of electrons, $N$: $$\sum_{b=0}^{N_b}f_b=N\tag{1}\label{occ}$$ where $N_b$ is the number of bands in the calculation. The occupancy is computed from $$f_b = f\left(\frac{\epsilon_b-E_F}{\sigma}\right)\tag{2}\label{smearing}$$ where $f$ is the ...


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

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 ...


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 ...


7

Your reference paper says that it subtracts the "absolute potential of the reference electrode" from the calculated values. I can't get to the citation that explains what reference electrode they are referring to, but the most common is the Standard hydrogen electrode, which is estimated to have an absolute potential of $\pu{4.44V}$. Subtracting ...


6

There is quite abit of work going on in this field right now. Take a look at what Rogers et al. are doing using MP2 to derive all aspects of a force-field. They are also working on a DFT implementation, since DFT is alot faster and computationally cheaper. I will assume that by interatomic, we are concerned with distant atoms interacting, and not near ...


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

Other answers are most welcome, but here I will outline the process through which I went to find a reference that you can cite for a hydrogen pseudopotential. Hopefully this will help you find references for other other elements, and to dig deeper on your own in cases where references for these types of things seem harder to find. I followed the URL in your ...


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 ...


5

DFT can be used in the field of electrochemistry to: predict reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions [1]. investigate electrocatalytic mechanisms [2]. model electron transfer across metal electrode/electrolyte solution interfaces [3]. These are a few examples ...


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

Expanding my comment to an answer, I believe the "trick" is incorrect. As far as I'm aware, the relationship between $\boldsymbol{r}$ and $\boldsymbol{\nabla_k}$ is $$\boldsymbol{r}\to-i\exp(i\boldsymbol{k}\cdot \boldsymbol{r})\boldsymbol{\nabla_k}\exp(-i\boldsymbol{k}\cdot \boldsymbol{r})$$ This would mean at least the middle equality of equation [...


5

"Inversion" is common in wavefunction-based quantum mechanics, for example the RKR inversion method which constructs a potential energy function based on information that can be obtained from spectroscopic experiments, such that a Hamiltonian using this potential, when fed through the Schroedinger equation, will give eigenvalue differences that ...


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 ...


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

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 ...


4

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 ...


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

Probably this recent paper by James Kirkpatrick, Aron Cohen and co-workers is getting closest to it: "Pushing the frontiers of density functionals by solving the fractional electron problem", Science 2021, 374, 1385. In it, they used Deep Learning to describe charge delocalization and strong correlation, resulting in the DM21 functional.


4

No, you don't have to put the pseudohydrogen at the mid-point of the bond. Putting it at the mid-point is an attempt to put it close to its optimal location, on the assumption that any X-H bond will be short (for a generic element X), and half of the X-X bond length is a good approximation. A simple alternative is to estimate how long the X-H bond would be, ...


3

From the WIEN2k manual: LAPWSO: lapwso includes spin-orbit (SO) coupling in a 2nd variational procedure and computes eigenvalues and eigenvectors (stored in case.vectorso) using the scalar-relativistic wavefunctions from lapw1 So, after running the inputs with -so, the output files will be named with so added to the end. For instructions about plotting ...


3

This has been answered very well already but just to add a practical note: For semiconductors, Vasp places the Fermi level at the valence band maximum ($\pm$ smearing energy) whereas Quantum Espresso or GPAW place it at the center of the gap. If you use another code then it's advisable to double-check what convention/algorithm it uses.


2

Constraint #16: Static linear response of the uniform electron gas This constraint is actually one of the 11 exact constraints that are also satisfied by the PBE GGA functional, as well as the TPSS Meta-GGA. If I'm understanding [1] correctly, the form of this constraint is: $$\lim_{k\to0}\gamma_x(y)=1+\frac{5}{9}y^2+\frac{73}{225}y^4-\frac{146}{3375}y^6+\...


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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