Questions tagged [density-functional-theory]

For questions about simulations using Density Functional Theory.

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23
votes
3answers
1k views

How big should a supercell be in phonon calculations?

Is 2x2x2 enough, or is larger needed? I know that a convergence test must be carried out, but increasing supercell size enormously increases computational time, and I am using the Python program ...
15
votes
1answer
232 views

Ab initio molecular dynamics to check material stability at finite temperature

I completed an ab initio molecular dynamic (AIMD) simulation in VASP for a hypothetical material. The simulation was done for 5ps with 1fs steps and the temperature was set to 300K using Nosé–Hoover ...
18
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2answers
206 views

Semi-infinite surfaces for adsorption: a valid approach?

I'm currently trying to surface reaction of small molecules on metal oxides in VASP. Several papers I've read have approached surface energy calculations in a variety of ways. I first began looking at ...
14
votes
1answer
173 views

How to convert from primitive to conventional after structure relaxation?

I did this in VASP but I guess this would be the case for any program. I have a structure that has a cubic conventional cell (a = 9.86 A). I found the primitive cell (a=b=c=6.97, alpha=90, beta=60, ...
13
votes
1answer
109 views

What is the speed/time required for DFT simulations?

I am currently designing a toy model to calculate an interatomic potential (for an infinite repeating lattice) this toy model needs to scale and perform better than DFT simulations. But sadly I have ...
14
votes
1answer
213 views

How can I scan the whole PES of a bulk structure?

I am not very familiar with scanning a PES for a bulk structure, and I'd be very appreciative for some suggestions.
21
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2answers
434 views

Create a new DFT functional from experimental data

This is my first time here and I think you can help me. I would like to try (just for fun) to create a new DFT functional starting from experimental data. Do you have any manual, procedure or ...
15
votes
1answer
254 views

Why is the band gap of graphene “opened” in this VASP calculation?

I used VASP to do the band structure calculation on Graphene. Originally I would expect that the Dirac cone can be observed in the K point. If we watched qualitatively, we can see this behavior. But ...
32
votes
1answer
1k views

Was Walter Kohn wrong about this?

In Kohn's Nobel Lecture, he claimed that: "In general the many-electron wave function $\Psi(\mathbf{r}_1,\ldots,\mathbf{r}_N)$ for a system of $N$ electrons is not a legitimate scientific concept,...
20
votes
2answers
474 views

Meaning of atomic positional parameters

I am trying to model some materials with DFT, so I intended to start out with the structure. However, some texts that present XRD data, use a term called atomic positional parameter. I can't seem to ...
11
votes
2answers
138 views

How to convert Unified Pseudopotential Format (UPF) into Ultrasoft Pseudopotential (USPP) format?

I am working on DFT code having plane wave along xy axis and bspline in z direction. For calculating the properties of TMD materials, spin orbit coupling must be included but i did not find fully ...
12
votes
1answer
72 views

What are the steps to prepare a solid-solid interface for adsorption studies in plane-wave DFT?

Please, how may one simulate a solid-solid interface that one can carry out adsorption studies from. I tried using virtual nanolab in Quantum ATK but didn't get a hand of it.
16
votes
1answer
182 views

The Hund's J - Why can this be quantified?

I was reading on the implementation of Hubbard 'U' in Density Functional Theory. The Hubbard 'U' can be thought of as a parameter that accounts for the on-site electron-electron interactions in ...
12
votes
1answer
206 views

How does the atomic arrangement look like in a compound that has partial/mixed site occupancies

I am working on modelling of a material that has partial/mixed site occupancies. For example consider the tetragonal (P4mm) compound $\ce{(Ba_{0.67}Sr_{0.33})TiO3}$. Following is the atomic ...
11
votes
1answer
52 views

How to generate relativistic pseudo-potential using Vanderbilt USPP that captures spin-orbit coupling?

I have used Vanderbilt USPP for generating a pseudopotential that does not include the spin-orbit term. But I want to have the spin-orbit coupling term included in the pseudopotential, to do ...
18
votes
2answers
130 views

Calculating binding energy between two systems

Regarding the calculation of binding energy of molecular absorption on graphene, for which this has been discussed before, the process of calculating $E_{AB}$ is fairly straight forward, i.e., it is ...
5
votes
1answer
74 views

Conductivity Computations of Molecular Wires [closed]

I am interested to compute the conductivity properties of some molecular organic wires in SIESTA. Since I am new in SIESTA, I would appreciate a guide for beginners to this field.
18
votes
1answer
331 views

Mathematical expression of SCAN (Strongly Constrained and Appropriately Normed) constraints in DFT

(This question is originally posted on physics stackexchange, but someone suggested me to post on this site, so there you go) I'm compiling the mathematical expression of SCAN (Strongly Constrained ...
12
votes
1answer
233 views

Can DFT be 'exact' in the limit of the uniform electron gas?

I was wondering if DFT (or specifically, LDA ?) can be exact in the limit of the homogeneous electron gas? In that case, shouldn't the self-interaction error perfectly cancel out? I realise that such ...
17
votes
1answer
153 views

What are the ways to ensure thermodynamic stability of a DFT modelled new structure?

One way of predicting the thermodynamic stability of a DFT modelled structure is to calculate the energy above convex hull, which was used as the criterion in The Materials Project database. For ...
17
votes
1answer
78 views

Converging the potential of a dilute binary alloy system using the Coherent Potential Approximation. What troubleshooting options could I take?

I'm trying to converge a dilute fcc alloy consisting of Cu and U, with the Uranium concentration at 0.01% and the lattice parameter of Cu, using Hubert Ebert's Spin Polarized Relativistic Korringa-...
13
votes
4answers
280 views

How to analyze the space group of a relaxed structure that has fractional site occupancies

I am using VASP package to model a material that has fractional site occupancies. An appropriate supercell (cif) was generated using supercell package and converted ...
13
votes
3answers
662 views

How would you explain Density Functional Theory to a high school student?

I find the experiments to be relatively easy to explain to my young cousins (who are planning to take scientific reserach as a career option). I find the theoretical works, especially the ones which ...
14
votes
1answer
52 views

Is there any way to semi-quantify antibacterial/antimicrobial properties of a material using DFT?

As we know, due to the current pandemic situation, people are getting more awake regarding hygiene and health. As such, there are many researches, both computational and experimental, within the ...
7
votes
1answer
58 views

Is there a relation between self-interaction errors and integer-discontinuities in Kohn-Sham-DFT?

I am wondering whether or not self-interaction error and integer-discontinuity in Kohn-Sham density functionals, are related to each other?
12
votes
2answers
312 views

Total spin and/or multiplicity for transition metal ions?

I am trying to input various charge states of various transition metal ion species into DFT calculations. The program requires an input of multiplicity = 2*(total spin) + 1. I have an undergraduate-...
15
votes
2answers
393 views

Hartree-Fock density vs Kohn-Sham density

Hartree-Fock density is free of self-interaction but lacks electron correlation effects, while the density from KS-DFT (using an xc functional or potential, both which are explicitly density-dependent ...
20
votes
3answers
876 views

Given that Kohn-Sham DFT is strictly a ground-state method (at 0 K), how is it sufficient to describe materials in real-life applications?

Kohn-Sham DFT appears to be so popular even though it is strictly a ground-state method - all calculations are done at 0 K. How then, is it so popular when describing materials that have real-life ...
12
votes
1answer
83 views

DFT vs tight-binding solutions for a Dirac cone in graphene: Why is the DFT version squeezed?

I'm a beginner at DFT calculations with VASP software. I use it to calculate the band structure of graphene and I obtain the following result: I used it to calculate the band structure of graphene ...
16
votes
2answers
194 views

Is the electronic band gap the only thing that is affected, when switching from standard KS-DFT to Hybrid functionals?

It is very well known that Kohn-Sham DFT underestimates bandgap. To get an accurate estimate of the bandgap, people often turn to Hybrid functionals (if they don't want to perform the actual ...
14
votes
1answer
588 views

Calculating HSE06 band structures on Quantum ESPRESSO

I am currently trying to figure out how to compute band structures for my system, using the hybrid functional HSE06. I'm doing this on Quantum Espresso. As I understand, there are a handful of ways to ...
22
votes
1answer
112 views

How do you calculate the “true” chemical potential in classical density functional theory?

In classical density functional theory, one traditionally calculates the chemical potential by taking the variational derivative, \begin{equation} \mu_{i} = \frac{\delta F}{\delta \rho_{i}} \end{...
10
votes
1answer
188 views

Spin–orbit interaction with DFT

I'm new to DFT. Does the DFT take into account the spin-orbit interaction? On the one hand, this is a relativistic effect, perhaps DFT does not take it into account. However, on the other hand, I ...
14
votes
1answer
308 views

Difference between Van der Waals (DFT-D and DFT-D3) corrections in ab-initio calculations

In Kohn-Sham DFT calculations, Van der Waals corrections are often implemented in the structure optimization calculations because the typical functionals such as LDA and GGA are found to not treat ...
11
votes
2answers
136 views

What is the mathematical condition that ensure that the self-consistent field (SCF) procedure must converge?

We know that there is convergence issue of DFT method on study real molecular system. It would be conducive if people could pre-determine if the SCF procedure is converge analytically before starting ...
14
votes
1answer
126 views

How the projected density of states is built

I'm new to DFT. I'm trying to figure out how projected density of states (PDOS) is built in solid state physics software package. I understand the basics of the density functional theory method: each ...
13
votes
2answers
209 views

How to calculate energy of a molecule in an aligned electric field?

So I am aware that many quantum chemical codes are able to add external fields to molecular calculations (for example to calculate polarizabilities). I also know that in some (eg. Orca) you have some ...
14
votes
2answers
430 views

Is there a performance difference when using fractional coordinates instead of Cartesian coordinates?

In general, the CIF file with the crystal structure uses fractional coordinates for setting the atoms positions. In this type of coordinates, the atom positions are relative to the cell parameters. ...
16
votes
1answer
146 views

Understanding the GTH Pseudopotential

I'm new to DFT. I want to make use of the GTH pseudopotential. I am trying to understand how this potential works. I read the article "Separable dual-space Gaussian pseudopotentials" by S. Goedecker, ...
21
votes
4answers
1k views

How to write my own density functional theory (DFT) code in Python?

I am a DFT user and at some point in the future, I would like to write my own DFT code in Python to help gain a deeper understanding of DFT. As mentioned in a previous answer people have written their ...
19
votes
2answers
1k views

How to “get my feet wet” in Density Functional Theory by simulating a water molecule using Python

I just saw I am a beginner in Density Functional Theory. What are some resources that could help me to learn the basics? and it reminded me that few years ago I'd asked How can I calculate the charge ...
19
votes
3answers
179 views

Hexagonal lattice volume EOS fit

I have a little problem concerning hexagonal volume fitting using VASP. I have followed these steps: Relax the structure from a given volume using ISIF =4 Copy ...
16
votes
3answers
824 views

I am a beginner in DFT. What are some resources that could help me to learn the basics?

I am a beginner in DFT. Please suggest me some good material to understand the basics.
15
votes
3answers
198 views

How to calculate homolytic bond dissociation energies?

An experimental colleague asked me how hard it would be to calculate homolytic bond-dissociation energies for different phosphonates which are involved in a Hydrophosphination. The compounds include ...
19
votes
1answer
107 views

Cancellation of Errors in density functional approximations or any wavefunction based methods

I am curious to find references for when is it TRUE that the error cancellation in approximate methods like DFT/MP2/etc. are valid and when they fall apart. For example: My understanding is (please ...
16
votes
2answers
151 views

State of the art in computational materials design

With the advent of more computational power than ever in the recent years, interest in in silico design of interesting compounds has grown as well. I am wondering about the state of the art for the ...
19
votes
2answers
117 views

Norm-Conserving or Ultrasoft, which pseudopotential to choose?

Ultrasoft pseudopotential (USPP), in general, requires a lower energy-cutoff to the plane waves when compared to Norm-Conserving ones (NC). On the other hand (at least with Quantum ESPRESSO) the ...
11
votes
1answer
67 views

Parallelization levels on Quantum ESPRESSO

Quantum ESPRESSO presents several levels of parallelization, including worlds, images, polls, and so on. However, there is a very intricate (and not clear to me) relationship between k-point sampling, ...
8
votes
1answer
57 views

How to generate a pseudopotential for a cation/anion?

Many DFT codes use pseudopotentials to represent the electron-ion interaction (as do most plane-wave codes and in contrast to so-called "all-electron" programs). There are several resources on the ...
8
votes
1answer
155 views

Is LDA better than GGA for elastic constants?

I have calculated elastic constants for Si, GaAs, and GaN using LDA and GGA in VASP. LDA is giving better results than GGA for elastic constants. Is there is a reason behind the good results by ...