Questions tagged [density-functional-theory]

For questions about simulations using Density Functional Theory.

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5
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1answer
59 views

What does GGA_COMPAT do in VASP and why is it recommended to set it to FALSE for non-colinear calculations?

Why is it recommended to set GGA_COMPAT = .FALSE. for non-colinear calculations? Could someone explain what GGA_COMPAT does?
9
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1answer
265 views

How to get atomic orbital coefficients in PySCF

I'm new to quantum chemistry software, so I'll try my best to explain my thought process. When running a DFT calculation, the software uses a linear combination of basis functions to approximate the ...
5
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1answer
148 views

Do the U values affect structure optimization with the DFT+U simulation to optimize crystal structure?

I am trying to do structure optimization of TiO2, since titanium is a transition metal, I need to use DFT+U to simulate the right band structure, but the electronic band structure would also affect ...
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0answers
69 views

The comparison of bandgap value of slab and bulk phase [closed]

I tried to calculate the bandgap of the TiO2-Rutile bulk phase, DFT(not +U) calculation shows the bandgap is about 1.8eV, which is roughly the same as the 1.78eV on the Material Project. When ...
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24 views

How would surface state affect the band gap value of semiconductors?

I am struggling to figure out the relationship between bandgap value and surface states. To my understanding, when considering the material that includes surface, there would be some surface states in ...
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0answers
46 views

Database of surface electronic structure

I'd like to compare my simulation result of a TiO2-110 surface's electronic structure, but all the literature shows too many different results, so I'd like to look for a database for surface ...
5
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1answer
73 views

Fermi energy lower than valence band maximum?

I am trying to analyze the density of electronic states of a $\ce{TiO2}$ slab, but the result I got from VASP shows the fermi energy is lower than the valence band maximum (VBM) by a small energy ...
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0answers
84 views

How does DFT decide the Fermi energy? [closed]

I am trying to figure out how density functional theory(DFT) decides the Fermi energy. As I understand, DFT could get the result of every orbital in the periodic system, then it could fulfill the ...
8
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1answer
136 views

Considerations when developing ab initio based interatomic potentials for classical molecular dynamics

I know that there are published interatomic potentials provided for most elements when using classical molecular dynamics tools such as LAMMPS. However, if someone were to use their own DFT ...
4
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1answer
25 views

Using pseudo-hydrogen in VASP results in the wrong format in CONTCAR

I am using VASP to do the simulation of a slab, I used some pseudohydrogen to passivate the bottom side of the slab. After the structure optimization calculation, the ...
5
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1answer
166 views

Calculated Redoxpotential is too far away from experimental value

I calculated the redox potential with Gaussian for hydroquinone with different combinations of functional/basis sets always values like 5.4V (B3LYP/6-311G+(2d,p) and SMD model). The experimental value ...
7
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2answers
166 views

Why don't we use the principal quantum number when building the projected density of state?

I am using VASP to calculate the projected DOS, but I am not sure why we aren't using the principal quantum number when doing the projection. According to the VASP wiki, LORBIT would control VASP to ...
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30 views

The workflow of VASP

I am trying to understand the workflow of VASP, so I make a simple diagram to show how VASP updates the related file during iteration. Did I make it right?
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62 views

How to set the AFM configuration in this doped monolayer system?

The figure below illustrates a 2x2x1 supercell of transition metal-doped CrI$_3$ monolayer. The red, green, and black atoms represent Cr, I, and transition metal atoms, respectively. I want to ...
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24 views

SQE (split charge equilibration) method [closed]

Is SQE (split charge equilibration) method a type of DFT method? Can someone help me in understanding this method !
7
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2answers
166 views

Does SCF calculation have to be done before calculating DOS?

The standard procedure to calculate DOS or band structure is: Structure optimization, SCF calculation, non-SCF calculation. According to the vasp wiki, it is because: Mind: Since the charge density ...
4
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1answer
25 views

When should I use dipole correction?

I am trying to understand the function of dipole correction in DFT. I find that there are two examples related to this topic on the VASP wiki. The first case is about CO on Ni 111 surface The input ...
3
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1answer
18 views

How to keep the freedom of an atom along a specific direction in VASP?

I am trying to relax my model's structure by letting some atoms could move freely along some specific direction, in VASP, I could set selective dynamics, but it seems I could only adjust the freedom ...
2
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0answers
34 views

What are the most advanced or best methods we know so far to try and find the exact exchange-correlation energy functionals for all matter? [duplicate]

Since the major problem with DFT is that the exact functional for exchange and correlation is not known, except for the free-electron gas (LDA from Thomas-Fermi model). The discovery of "the"...
5
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1answer
34 views

What is vaspcmd in job.sh of the official VASP examples?

There is job.sh script in a lot of official VASP examples, for example: Graphite MBD binding energy. it is used to help submit job on supercomputer But in the script, it usually uses ...
7
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1answer
95 views

What is the principle of generating INCAR tags in ASE?

I am wondering if there is a detailed explanation about how ASE chooses the INCAR tags, since VASP already provides default values for a lot of tags, does ASE choose the default values? There are a ...
8
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1answer
123 views

When to use pseudohydrogens to passivate a slab?

I often see some papers that don't use pseudohydrogen passivating the slab's bottom side when doing DFT simulation on the metal oxide's surface, for example, the following description is from a paper ...
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82 views

Why is there a deviation between the experimentally calculated values and theoretical done by DFT calculation? [duplicate]

If there was a deviation between the experimentally calculated values and theoretical determination done by DFT calculation, then what is the reason?
5
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1answer
53 views

Where should I put the pseudohydrogen?

Usually when passivating the slab's bottom side with pseudohydrogen, I would optimize the structure of the bulk, cut the bulk and get the slab model, then I would add the pseudohydrogen at the ...
5
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0answers
55 views

Validity of estimating electron relaxation time using the smallest cross section of a structure

I am studying nano-scale constrictions in semiconductor materials. Imagine two nano-particles joined together by a single point of crystalline contact to look like an hour-glass filled with sand. ...
6
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1answer
31 views

Some questions on case.energy and case.energyso in WIEN2k

In WIEN2k, the subprogram lapwso will generate both case.energy and case.energyso files. ...
8
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1answer
98 views

Citing a pseudopotential

I conduct my DFT calculations using the SIESTA DFT package. For this, I obtained the pseudopotentials from this site which says that they have obtained the data from the Abinit database. However I do ...
11
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2answers
449 views

Why the band structure of a slab is not smooth?

I am trying to calculate the band structure of a TiO2-Rutile slab, but the result I got is not very smooth, and the result shows it is an indirect gap material, but the bulk phase of TiO2-Rutile is a ...
7
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1answer
48 views

SIESTA error: must be in PAO.Basis, (it has semicore states)

I keep getting this error with Siesta every time I do a calculation, ...
5
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2answers
141 views

How can I get started on DFT?

This question is simply: How can we use DFT in the field of electrochemistry? What are the best resources (Books, reviews, papers, software ...) to learn about using DFT in electrochemistry? ...
6
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1answer
37 views

How to determine the right k-points for the band structure of a slab?

I am trying to compare the band structure of a slab and the bulk phase, just like the example on the VASP wiki. In the example of VASP, it calculates the band structure of the slab of Ni metal. A slab ...
6
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2answers
48 views

DOSCAR file during structure relaxation is useless?

I am trying to figure out why the DOSCAR file during structure relation is useless, for the last step of structure, the atoms' positions are almost fixed, is that the same as a static calculation? But ...
8
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1answer
109 views

What is the detailed difference between Vasp_std and Vasp_ncl?

I am trying to figure out the difference between vasp_std, vasp_gam, and vasp_ncl. I find some related details on the Vasp forum, I think vasp_gam makes some customized optimization for the gamma-...
6
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1answer
165 views

How to choose the best SIGMA value in VASP?

I am trying to test how to choose the SIGMA value in VASP, so I did a little bit of test. Here is the input file: INCAR ...
6
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1answer
95 views

What are the electronic steps and ionic steps in VASP?

I am getting confused about what the electronic step and ionic step are in VASP. I used to think that finishing the following loop one time is called an electronic step. but I am not sure how to ...
6
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1answer
50 views

Is it necessary to do self-consistent calculation after structure optimization?

When calculating the band structure of a material, I have been told that at first, I need to do structure optimization to get a stationary structure(1st step), and then keep the atoms' position fixed, ...
6
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1answer
129 views

Why does it take 5-10 steps to obtain reasonable orbitals during initialization in VASP?

I am trying to figure out the initialization process of VASP, and I found an INCAR tag NELMDL, on the wiki page of NELMDL, it is said that VASP requires 5-10 steps to obtain the reasonable orbitals. ...
5
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1answer
60 views

Different between ENCUT and ENAUG in the VASP setting?

I am trying to figure out the difference between ENCUT and ENAUG in VASP settings, I found the following description on the wiki of VASP. ENAUG specifies the cut-off energy of the plane wave ...
7
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1answer
35 views

Are CHARGE=2 and INIWAV=1 contradictory in the VASP setting?

When initializing the charge density in VASP, I set the parameters as follows: ...
8
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1answer
37 views

Does the INIWAV tag in VASP assume that the wavefunctions are plane waves?

I am trying to figure out how to set the initial wavefunction in VASP, on the VASP wiki page of INIWAV, it is said that VASP would fill wavefunction arrays with random numbers. ...
7
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34 views

Which specific perturbation do we consider applying to the potential when defining the energy derivative of the log derivative?

Norm-conserving pseudopotentials are defined such that the energy derivative of the log derivative of the real and pseudo wavefunctions agree at $r>r_c$. I understand that although valence ...
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103 views

How to treat delocalized pi bonds in DFT

For a structure such benzene, the pi bonds between the carbon atoms are said to be de-localized. Therefore the electrons are expected to be exchanged in between individual C-C bonds. When dealing with ...
4
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1answer
54 views

How to understand the energy derivative of the logarithmic derivative of a wavefunction when deriving norm-conserving pseudopotentials

The 4th desirable property of a norm-conserving pseudopotential given by Hamann et al is for the 'logarithmic derivatives of the real and pseudo wave function and their first energy derivatives agree ...
13
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1answer
128 views

Physical interpretation of k-points

What is the Physical interpretation of the k-points used in ab initio calculations? Why must the number of k-points be optimized rather than just using some large fixed number of them?
6
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1answer
138 views

How does electronic iteration work in a VASP relaxation calculation?

I am a beginner using VASP to do material modeling, and I'm confused about the following questions: How does the electronic iteration work in VASP relaxation calculation? Is the electronic iteration ...
8
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1answer
268 views

Study of supercell after creating vacuum

For studying a bulk supercell, after creating some vacuum in it, I'm confused about how to relax it. With VASP, I can do relaxation using NSW = 0 and ...
6
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2answers
88 views

Relation between lattice parameter and energy for relaxations

We have been wanting to do relaxation of lattice parameters as a function of the energy. I want to know the mathematical expression for this, and also the theoretical background related to this method....
8
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1answer
83 views

what do I need to change about typical VASP input file to calculate the energy of a single atom?

I'm a little confused how to make a VASP input file to calculate the energy for the isolated atom: Do I need to change the lattice parameter of my unit cell or can keep the value for the whole system?...
6
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0answers
28 views

TIP5P model for water in RISM [closed]

I want to use the implementation of ESM-RISM from the Otani laboratory using Quantum Espresso. For my system I ask for $H_2O \; (H^+Cl^- \; 1M \; aq.)$, following the literature the best way to ...
6
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0answers
47 views

Cutoff length for vdW correction in DFT calculation

In classical MD simulations of lattice structures, whenever we are trying to incorporate vdW corrections to the atomic force calculation, we need to set up cutoff length beyond which the vdW ...

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