# What is Dft: Definition and 120 Discussions

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1. ### Engineering MATLAB Exercise on DFT

Good morning, I have an exercise with two points that gives a Matlab mat file in which are contained the samples of two measured signals and the variable fs indicating the sampling frequency A)Signal 1 consists of two sinusoidal signals of which you are asked to calculate: -the duration ⇒ I...
2. ### Software for calculting DFT and Raman spectra for Windows

Dear Friends! Please recommend a program for performing DFT calculations on Windows. It should also be capable of calculating Raman spectra. It would be highly desirable if the program will be free. Sincerely, Aleksandr
3. ### A How does DFT handle degenerate eigenvectors?

I have a question about how DFT (density functional theory) handles degenerate states. The Hamiltonian in DFT is a functional of the electron density defined via ##n(\mathbf{r})=\sum^N_{k=1}|\psi_k(\mathbf{r})|^2##. However, say I have a pair of degenerate states. Then any linear combination of...
4. ### I Question on invertibility in finite fields

Hello all, I have here an excerpt from Wikipedia about the discrete Fourier transform: My question(s) are about the red underlined part. 1.) If ##n## divides ##p-1##, why does this imply that ##n## is invertible? 2.) Why does Wikipedia take the effort to write out the ##n## as ##n =...
5. ### Implementing “Ab initio” approach in molecular mechanics method

Experts in physics, physical chemistry and organic chemistry are invited to the thread. I work in the field of quantum chemistry, and have plans of implementing molecular mechanics calculations in my program Chemcraft. Some people say that molecular mechanics can work rather well in some cases...
6. ### A Learning DFT: Inhomogeneous Electron Gas (Hohenberg) Question

I'm reading through Hohenberg's seminal paper titled: "Inhomogeneous Electron Gas" that help lay the foundation for what we know of as Density Functional Theory (DFT) by proving the existence of a universal functional that exactly matches the ground-state energy of a system with a given...
7. ### I Number of lines in Band Structure

The general idea is that, number of bands=number of lines. For instance, AlAs primitive cell (one of each atom per cell) has 13 bands; that is outer most shells have 13 distinct energy values. (Al: one 3s + three 3p, As: five 3d + one 4s + three 4p) But when I calculate Band Structure through...
8. ### What is the difference between DFT and TD-DFT in Gaussian 09 software?

Homework Statement:: difference between DFT and TDDFT in Gaussian 09 software? Relevant Equations:: hello , I hope this message finds you well Please I want to know what is the difference between DFT and time‐dependent density functional theory TD-DFT and why we use it in the measurement of...
9. ### I Potential Energy of an Electron-Nuclei Interaction in DFT

In density functional theory (DFT), electron density is a central quantity. Because of this, we want to calculate electron - nuclei potential energy as functional on electron density. If we know how potential energy varies across space, we can calculate this functional with plugging particular...
10. ### A Fourier Transform of an exponential function with sine modulation

I want to know the frequency domain spectrum of an exponential which is modulated with a sine function that is changing with time. The time-domain form is, s(t) = e^{j \frac{4\pi}{\lambda} \mu \frac{\sin(\Omega t)}{\Omega}} Here, \mu , \Omega and \lambda are constants. A quick...
11. ### Signal Processing: DFT spectrum of sinusoid signals

Hi, I was recently reading about the discrete Fourier transform and its application to a basic sinusoidal signal. If we know that it has an integer number of cycles in ## N ## samples (and thus no leakage), why would there be two peaks in the spectrum: one at ## m ## and another at ## N - m##...
12. ### A DFT calculations for metal oxide semiconductors and graphene oxide

I am doing experimental research on metal oxide semiconductors and graphene oxide, I also want to do some theoretical analysis. I am new in theoretical, so I need guidance how to proceed with the simple calculations, like DFT. Is there any authentic book with examples? and also is there any...
13. ### Engineering How to compute the DFT of a cosine

Hello, This is a more general question than anything, but I am curious how to compute the DFT of a cosine wave. Somebody tried to explain this to me as follows: start by trying to find an x(k) who's IDFT equals cos(2*pi*n/N). x(k) = (N/2) * (dirac-delat(k+1) _ dirac-delta(k-1)) only has values...
14. ### Second programming language to get under the belt: python vs C

Hello everybody, I am a master student in Theoretical Chemistry and I am working in the DFT realm. Both TDDFT and DFT applied to extended systems (eg. using QUANTUM ESPRESSO). Of course I work with these softwares from a end-user point of view, not as a developer. But anyway, even if some of...
15. ### A DFT Meaning of k's Greetings: Understanding Wavelength & Interval Length

Greetings! I am getting started with Python's Fast Fourier Transform, and I noticed a missunderstanding on my part. I always thought that the k's in the expression of the DFT resemble actual wavenumbers ## \frac{2\pi}{\lambda}## that form the waves the signal is composed of. But the actual...
16. ### CuO Unit Cell Optimization w/ DFT: Tips & Advice

Hello everyone Currently, I am trying to optimize CuO unit cell and CuO(1 1 1) through using gaussian interface and DFT calculation method. can somebody guide me the appropriate parameters in order to do that because so far all the basis sets and functional have failed to work so if you could...
17. ### Do you think artificial intelligence and DFT codes can be merged?

Hello! Do you think artificial intelligence and DFT codes can be merged? Is it time for the intelligent DFT?
18. ### A Diagonalization of Hubbard Hamiltonian

Hi guys! I am starting to study Hubbard model with application in DFT and I have some doubts how to solve the Hubbard Hamiltonian. I have the DFT modeled to Hubbard, where the homogeneous Hamiltonian is  H = -t\sum_{\langle i,j \rangle}\sigma (\hat{c}_{i\sigma}^{\dagger}\hat{c}_{j\sigma} +...
19. ### DFT for Helium using Mathematica

I'm trying to solve this problem with Mathematica. Im not a Mathematica expert, but my program works perfect when vhartree={0,0,..,0}. This is the program I wrote: Clear[u, poisson, vhartree] h = 10^(-2);(*step integration*) rmax = 20; rmin = 10^(-30); Z = 2; (*atomic number*) points =...
20. ### A Viscosity from DFT (VASP) using the Green-Kubo relation

Hello! In this paper https://pdfs.semanticscholar.org/e8a2/02f25555cd8c4f947bbbdff5a61a0ea0efd2.pdf the authors use VASP to determine MgSiO3 viscosity using the Green-Kubo relation ## \eta = \frac{V}{3k_{\rm{B}}T}\int_{0} \left<\sum_\limits{i<j}\sigma_{ij}(t+t_{0}).\sigma_{ij}(t_{0})\right>dt##...
21. ### A What resources are available for using DFT to study 1D materials in physics?

hello dear physicists I will work in my thesis on 1D materials using DFT as a numerical method to find the properietes of these 1D materials I would be very happy if someone can help me with references (books, links, articles, vedios ...) that could help me to advance in my work Thank you
22. ### DFT vs. DTFT: Understanding the Difference

Homework Statement I'm kind of confused between DFT and DTFT. Here is my understanding: Okay, so let's say we have time domain, continuous, analogue signal from a sensor - ##x(t) ## 1. We sample this signal, giving us something like the following with an impulse train Now this is a...
23. ### Matching Discrete Fourier Transform (DFT) Pairs

Homework Statement [/B] I am trying to match each of the following 28-point discrete-time signals with its DFT: Set #1: Set #2: Homework EquationsThe Attempt at a Solution Set #1 We have already established (here) that: ##Signal 1 \leftrightarrow DFT3## ##Signal 4 \leftrightarrow...
24. ### Discrete Fourier Transform (DFT) Matching

Homework Statement Match each discrete-time signal with its DFT: Homework EquationsThe Attempt at a Solution I am mainly confused about Signal 7 and Signal 8. Signal 1 is the discrete equivalent to a constant function, therefore its DFT is an impulse (Dirac ##\delta##), so it corresponds...
25. ### Admissions DFT & VASP Experience for Graduate Programs in SS Physics

How useful is experience with software like VASP as an undergraduate when applying to graduate programs in solid state physics?I'm not asking if they expect you to know how to use it,but can it come across as something important in the application?
26. ### What are the key concepts and abbreviations related to DFT and HF theory?

I hope you guys can help me understand these concepts better, did'nt find so much on google. This is from an old exam, I am preparing for my exam and I feel that I need to have an simple understatement of these. Thanks a lot! 1. Homework Statement Explain the following concepts and...
27. ### I 2 basis sets, for which one is DFT calc faster?

Hello fellow physicist, I'm new on this subject, I hope that you can help me clear some stuff out. If I have 2 basis sets; A containing 100 basis functions and B containing 50 basis functions, for which one of them will a DFT calculation be faster? Also, is a DFT calculation using basis sets...
28. ### A DFT Code for Finite-Temperature Calculations

I'm interested in calculating density of states (DOS) and band structures for metals in which the electrons have a finite temperature (up to ~ 3 eV). I've read several papers indicating that the DOS shifts with electron temperature, and that DFT can be used to calculate finite-temperature...
29. ### A DFT modelling of graphene, fullerenes, nanotubes etc

Hi all. Does anyone here have any experience modelling graphitic materials using DFT based software (eg VASP)? I am looking to utilise some simulation results in conjunction with applied research on these materials. The focus is really the applied stuff, and I have very little background in...
30. ### Number of subdivisions in a Riemann integral (DFT)

Homework Statement This is a combination of two questions, one being the continuation of the other 3) Calculate the DFT of the sequence of measurements \begin{equation*} \{ g \}_{k=0}^{5} = \{ 1,0,4,-1,0,0 \} \end{equation*} 4a) Draw the DFT calculated in question 3 on the complex plane. 4b)...
31. ### Find the Answer to x[n] in a 2-Point DFT

Homework Statement The 2-point DFT of x[n] is given by the expression X[k] = 2δ[k], for k = 0, 1. Homework Equations What is x[n]? The Attempt at a Solution i know the answer but i don't know how they calculate it because a delta pulse is only one at k=zero. x\left[n\right]=\frac{1}{N}\sum...
32. ### Understanding N-Stages of FFT & DFT Formula for Frequency Domain

1) How many stages are in N numbers for a FFT? I know N=8 has 3 stages and N=4 has 2 stages ? 2) The DFT formula converts a signal from the time domain into the frequency domain. Is this done by comparing x[n] against signals known as sinusoidal basis functions? are e-j2 pi kn/N sinusoidal...
33. ### A Getting Started with CASTEP: Installing & Configuring for DFT Calculations

Hi guys, I'm trying to use the CASTEP software for dft calculations. I have plenty of documentation on how to use it once it's all up and running, however I don't know how to get to that point :/ I'm not the best with linux but I've manually installed programs before through tarballs but here...
34. ### Domain of a discrete fourier transform

Homework Statement The (computing) task at hand is to take a function f(x) defined at 2N discrete points, and use the Discrete Fourier Transform (DFT) to produce F(u), a plot of the amplitudes of the frequencies required to produce f(x). I have an array for each function holding the value of...
35. ### Density Functional Theory (DFT) textbooks

I'm starting out on DFT right now. I'm an experimental Physics student, so I'm not very familiar with theories. Can you recommend any good textbooks or resources that I can utilize for my study?? Thanks in advance.
36. ### A Gaussian 09 Output file_DFT Calcuation

Hi, I am trying to understand the functional form of B3LYP from the Gaussian output file. I have tried to relate the details in the output file with the functional form of B3LYP. But I am not sure what certain terms correspond to. I have mentioned below the details. Can you pleaese help me. In...
37. ### A Kohn-Sham vs Gell-Mann & Low correspondence

Both the Kohn-Sham theorem and the Gell-Mann & Low theorem provide a way to map an interacting condensed matter system on a non-interacting system, each in their own language. The Kohn-Sham approach does so for density functional theory, calculating the properties of a system from the...
38. ### A How to find vibrational frequency for specific bond? (DFT)

Hello, I performed an optimization and frequency calculation at the ub3lyp 6-311g level of theory. The compound has the formula C39H41PS2. Using Molden, I am having a hard time identifying the frequency corresponding to the stretching of a phosphorus carbon double bond present in the molecule. I...

40. ### A Why does my chosen basis set for W crash (DFT)?

Hello, I am trying to optimize a molecule with the chemical formula C60H52O18P4S4W2. I have tried using different Basis Sets on my input file but I keep getting the following error: " Standard basis: Aug-CC-pV5Z (5D, 7F) Atomic number out of range in CCPV5Z. Error termination via Lnk1e in...
41. ### Why is there a need for correlation hole?

I have been reading about the physical meaning of exchange-correlation hole and this is what I have found so far: Exchange hole - attributed to the spin of the electrons. Electrons of same spin will not occupy the same orbital because of Pauli Exclusion Principle. This leads to the lowering of...
42. ### How big should a defected supercell be? DFT

When studying defects using plane-wave basis density functional theory, it is necessary to ensure that the size of the supercell in which the defect is located is large enough to ensure that there is no interaction between the defect in question, and the periodically repeated defects that are a...
43. ### Mastering DFT: Calculating Band Structures for Heterostructures

I'm teaching myself how to do DFT for my master's project, where I want to use it for calculating band structures for different heterostructures. Now to learn DFT I am on one hand reading a book on the basic theory, on the other hand using different freeware packages to try and calculate the...
44. ### Computational Can you suggest books that will help understand DOS graphs?

Can you suggest books or any resource that will help understand DOS graph discussions in journal articles? I am reading DFT results. I try very hard to understand how their discussion is related to DOS graph presented, but I can't no matter how I reread it over and over again. :frown...
45. ### Inverse Discrete Laplace Transform

Hi, I have an idea which when tested looks like its clearly flawed. I am hoping someone can tell me where my procedure is flawed, or point me to some other theory that has already done something similar. The first two are the laplace transform. The third line is the Fourier Transform. The...
46. ### MATLAB Transforming Complex Exponential to Discrete Vector Form

Hi, I want to transform a complex exponential with quadratic phase to discrete form, in other words to a vector form. can anyone help me with that? Thanks
47. ### Calculating DFT for specific polynomial

Hello everyone, This seems like a simple problem but I get the impression that I'm missing something. 1. Homework Statement Given the values ## v_1,v_2,...,v_n ## such that DFTn ## (P(x)) = (v_1, v_2, \ldots, v_n) ## and ##deg(P(x)) < n##, find DFT2n## P(x^2)## Homework EquationsThe Attempt...
48. ### Improving Frequency Resolution with Window Functions in FFT Calculations

Okay I have a question involving calculating the FFT of a signal from a sensor. I have simulated many different scenarios in MATLAB of various noise characteristics involving the signal. I want to take the FFT of a noisy signal. As long as my expected input signal has a higher amplitude than...
49. ### How to get specific frequency and time values for .wav file

How to get specific frequency and time values for .wav file and export the values as .csv? Problem: I need to analyze this music .wav file, particularly for its frequency, amplitude, and pitch over specific intervals of time. Is there any easy to use software and steps I can take that can...
50. ### Running Mbj potentail problem in dft wien2k sofware

i am runnig mbj calculation using wien2k as mentioned in user guide but can't be able to successful run,ambiguity in scf's so can anyone please guide me clear steps for mbj calculation. and modifications