Questions tagged [density-functional-theory]

For questions about simulations using Density Functional Theory.

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8
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0answers
47 views

Inconsistent data of chemical potential from different papers

I am looking for chemical potential from literature: ...
7
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2answers
208 views

acceptable range of stretching of a lattice unit cell when we do the heterostructure

Currently, I have to put two different materials together to form a heterostructure. I've already transformed the unit cell of material A from trigonal to cubic. Now the unit cell has a rectangular ...
12
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1answer
162 views

How to explain Density Functional Theory results to an experimentalist?

When we present our Density Functional Theory simulation results e.g. lattice parameters, stacking fault energies, band gaps, etc. to people who are experimentalists then the very first question ...
7
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2answers
63 views

Combined effects of strain and doping on the electronic structure of semiconductors

I've read papers in DFT studies on band gap tuning in semiconductors and the usually studied methods are either through doping or application of external strain. But it is always just one or the other....
7
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1answer
48 views

Different energies for two different geometries where only the monolayer c lattice constant is different?

I carried out a simple single-point PBE calculation on two different graphene monolayers (only c is different) using VASP with 12x12x1 k-grid. Geometry 1: ...
10
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2answers
157 views

Band structure with additive dispersion methods

Let's say I optimized geometry with PBE+D3, does it matter if I make single point bandstructure calculation with just PBE or PBE+D3 since D3 is a additive method.
13
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5answers
241 views

DFT software package for a beginner?

I would like to do start doing some basic calculations using DFT. I am interested in solid state physics of strongly correlated materials in 3D (for example high temperature superconductors) or 2D (...
7
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0answers
57 views

What are the advantages/disadvantages of QMC over ACFDT-RPA?

I had some experience with the ACFDT-RPA, despite its promising features it's not practical most of the times, and for that reason, I'm looking for a new method for reference calculations. QMC is the ...
10
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1answer
159 views

Are QMC calculations practical for periodic structure calculations?

Even some DFT methods or beyond DFT methods such as MP2 calculations are computationally very expensive today even with clusters. If you compare QMC (quantum monte carlo) calculations with them, are ...
11
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3answers
175 views

How to determine whether we can put two semiconductor thin film together?

I am beginner of material modelling that's why my question may be basic and I have no direction. If we want to see whether two crystal layer(semiconductor) can be put together, can the DFT calculation ...
9
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2answers
51 views

Analytic Hessians for meta-GGA functionals

In many of the free/open-source QM programs like GAMESS, ORCA, NWChem, the calculation of analytic hessians is not possible when a meta-GGA density functional is used, like M06-2X. My first question ...
9
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1answer
76 views

Monkhorst-Pack, Gamma-centred and gamma only

Sorry if this question is too simple for this community but I still couldn't find an answer to it. Do gamma-centred and gamma only grid mean the same thing? If not, what's the difference? How do they ...
7
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1answer
59 views

What are the methods for calculating interaction energies of layered materials, and where are they implemented?

What are the available methods for the calculation of reference interaction energies of layered materials and in which codes they are implemented? For example, I know RPA-ACFDT in the DFT concept. But ...
9
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0answers
56 views

Problem related to the calculation of elastic constant with VASP5.4.4

I am using VASP5.4.4 to calculate the mechanical properties of the cubic phase SnTe. The structure can be found in the materials project website. In detail, I use the following INCAR to calculate the ...
8
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2answers
101 views

Evaluate the orbital magnetic dipole moment within the PAW sphere

In general, the orbital magnetic dipole moment operator is defined as: $$- {e\over2} \mathbf{r \times v} \tag{1}$$ $\mathbf{v=p}/m$ is true only for simple Hamiltonian like $-1/2\ \nabla^2+V$, but not ...
9
votes
1answer
137 views

Is there a software that has implemented forces of BSE calculations for solids?

I'd like to investigate the structural relaxations of the excited state for periodic systems. It seems BSE is a practical method that has already been implemented in many codes for static calculations....
11
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1answer
70 views

adaptive k-mesh

Are there any known codes or scripts to generate an adaptive k-mesh for DFT calculations? For example, I'd like to make the k-mesh near the $\Gamma$ point (e.g., half the zone) more denser ($20\...
13
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5answers
2k views

Can I use two different codes?

Can I use one code for geometry optimization and another code for a single point calculation on that optimized geometry? Can one publish results like that?
7
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1answer
70 views

Approaches to understanding different relative energies between semiempirical and DFT methods

I am studying Pd(II) metal-organic systems and have used GFN2-xTB (with implicit solvation) for high-throughput optimisation of candidate structures. I use the relative energies (from xtb) of ...
5
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1answer
51 views

How to implement scissor-correction in VASP to study optical properties?

It is mentioned here that It is possible to correct for the DFT band gap error by introducing an empirical scissors correction, which is effectively a rigid shift of the conduction band with respect ...
11
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3answers
272 views

External magnetic field in VASP?

I have never found any tags or documentation regarding external magnetic field in VASP but in different well known published articles$^1$, the implementation of the external magnetic field by VASP has ...
8
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1answer
105 views

How to perform virtual crystal approximation calculations in VASP?

Can someone share appropriate tags and methods to perform virtual crystal approximation calculations in VASP?
9
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1answer
49 views

What is exfoliation energy per unit area?

I see in many sources the exfoliation energy defined with the unit $meV/A^2$ I frankly don't understand how it's calculated and I couldn't find any mathematical formula for that. its unit confuses me. ...
9
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1answer
159 views

Can the fermi level of a semiconductor be below the valence band?

I was looking at the data on this page for a NiWO4 calculation. I can see unoccupied bands near the fermi level that appear to be below the band gap. What does this actually mean physically or is it ...
8
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1answer
112 views

How to fix Error Link9999 in Gaussian

I have a problem with geometry optimization in Gaussian software with M062X method. The structure was smoothly optimized using M062X/Gen. Nevertheless, I need very accurate results so afterwards the ...
4
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0answers
46 views

What are the some other ways to handle BSSE rather than counterpoise correction and using larger basis sets? [closed]

What one can do to eliminate basis set superposition error (BSSE) if there is no chance to use larger basis sets or counterpoise correction.
12
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2answers
347 views

What is Surface termination and how to make surface termination of a crystal structure?

I want to calculate the surface energy of CuO and for that, I've asked CuO (monoclinic) crystallographic plane and their relaxed surface energy. But for the lack of knowledge of the surface ...
11
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2answers
190 views

Effect of ISYM tag on Partial Density of States (VASP version < 6)

This is from the entry for LORBIT in the VASP Wiki. For LORBIT >= 11 and ...
8
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0answers
61 views

I ran TDDFT on some small molecules. For some files, the ORCA output files cut short and contain no absorption data. What could be the reason?

Below is just the end of the output file, and the top of the input file. This is for the molecule FDICTF. The calculation did not run out of time. I ran a batch of 15 molecules, all with same ...
10
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1answer
386 views

Which method gives the most accurate electron density, and how can it be verified experimentally?

I read a paper from Science (DOI https://doi.org/10.1126/science.aah5975), “Density functional theory is straying from the path toward the exact functional”. They were trying to make a point that ...
7
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2answers
62 views

Input constraints while comparing different DFT calculated properties for the same material but different atomic arrangement

Let's consider a fictitious AB type binary alloy. AB is known to exist in a B2 type ordered ...
9
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1answer
168 views

How to start with structural defects in monolayer?

First of all, thank you for your help! You are so helpful every time. I would like to calculate the influence of the structural defects on the electronic structure in the HfS2 monolayer. With VESTA ...
11
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2answers
124 views

Which properties of a material calculated using density functional theory are affected by smearing?

Smearing (smearing width $\sigma$, to be precise) has always been confusing. I understand what it is but can't pin it down when it comes to DFT calculations. Should it be converged like k-points and ...
11
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2answers
139 views

Permissible amount of forces on ions in a relaxed structure

An equilibrium structure would be at a local minimum but that doesn't happen with all relaxation calculations. Relaxed structures tend to have some resultant forces on ions. What amount of these ...
8
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2answers
87 views

k-points convergence for spin polarised vs non-polarised DFT calculations

Spin-polarised calculations come at a computational cost and I tend to take ISPIN = 2 only after I'm done with a proper initial relaxation of the structure, so the ...
11
votes
1answer
174 views

How would you report the lattice parameters of an alloy, modelled using a supercell

There is a whole lot of literature behind ab initio modelling of alloys with statistical site occupancies. These include VCA, SQS, CPA, supercell approach, etc. This question does not aim to compare ...
17
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2answers
913 views

Logic of published geometry optimization results without checking phonons

I see that in many papers where optimization results presented, they do not mention phonons and their force convergence criterion is 0.01 eV/A. Can you publish a result like that? And is not this ...
7
votes
1answer
72 views

DFPT vs Finite Difference Method [duplicate]

Which one is better in case of a phonon calculation and does DFPT also have small negative frequencies near Gamma due to the numerical noise? What are their advantages and disadvantages?
11
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2answers
184 views

Band Structure mBJ-LDA using Vasp

I tried to create a band structure for mBJ-LDA using VASP. I found information that the KPOINTS file should be created according to the "0-weight (Fake) SC procedure" (as for HSE) I have a ...
13
votes
1answer
100 views

What is a “Charge Density Wave” and how is it related to the imaginary frequencies on phonon dispersion bandstructures?

I see that for some structures that have imaginary frequencies, they mention CDW, but I could not find any reasonable explanation about the relation between them. Does CDW cause instability in the ...
8
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2answers
102 views

How to make a good HSE06 bandstructure for TMDC monolayer material (e.g.: WS2) in terms of VASP calculation INPUTS?

I want to perform HSE06 band-structure calculation of a WS2 monolayerwhich has the following POSCAR with ...
8
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3answers
300 views

How to generate KPOINTS file for HSE06 band-structure calculations?

I have had a look at the example of Silicon provided here in the WIKIVASP website, and I noticed that the KPOINTS file used for ...
8
votes
2answers
257 views

Is there a possibility to change the initial crystal system during full geometric relaxation?

One of the challenges of modelling completely new material using DFT is the selection of initial geometry. Usually, an experimental structure (cif) which appears to be similar to the new material in ...
10
votes
1answer
148 views

Is it okay to take a cubic Monkhorst-Pack k-mesh for a non-cubic supercell in a DFT calculation?

Assume that the numbers of k-points in each direction are sufficient. Example - Lengths of lattice vectors: $2\: 2\: 4$ Required k-mesh: $9\times 9\times 5$ Would a $9\times 9\times 9$ k-mesh work? ...
9
votes
1answer
102 views

Muonic Modeling

I have seen an article[1] that describes a HF approach to working with exotic muonic atoms. I am curious about how far DFT has been extended for modeling muonic chemistry. Are there psuedopotentials ...
7
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0answers
63 views

ASE's fmax and FHI-AIMS's relax_geometry bfgs “tolerance”. Are they same?

They look like the same thing, but their definitions are confusing. Additionally, while it's possible to use 'tolerance'< 0.05 in AIMS easily, in ASE, some structures never optimize if you choose ...
4
votes
1answer
75 views

How much energy difference could be used to distinguish optimized results from different initial geometries for the same structure?

Is there a threshold? Could I say that my final geometries are the same with 'energy difference = 0.05 meV/atom' ?
9
votes
1answer
64 views

“Input forces are not enough to calculate force constants” Phonopy error?

I am doing a phonon calculation for a phase using a 2x2x1 supercell (after geometry optimization of the structure). I am getting an error when computing the DOS and band structure after computing the ...
13
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2answers
565 views

What does it mean if a strictly optimized experimental geometry presents imaginary frequencies?

Let's say you optimize an experimental geometry of a structure with very large basis sets, strict optimization settings with different XC functionals and after calculating phonons of this structure ...
9
votes
1answer
122 views

Freely available constrained DFT implementations

What free quantum chemistry packages that have constrained DFT (CDFT) implemented? A google search does not reveal anything, perhaps there may exist a better resource to find the packages.