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Questions tagged [density-functional-theory]

Density functional theory (DFT) is a quantum mechanical model used to estimate the electronic structure of molecules and condensed matter. In broad terms, DFT works by treating all the electrons in the system as a single electron density, and computing physical quantities of interest as functionals of that electron density.

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Theoretical explanation on calculating 'total magnetic moment' and 'absolute magnetic moment' in DFT (Quantum Espresso)

Could someone please provide a comprehensive theoretical explanation of how to calculates the total magnetic moment and absolute magnetic moment of a material in quantum mechanics (DFT approach) ?.
Thejan Hasaranga's user avatar
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The criterion about maximally localized Wannier function (WF)?

I heard that as the value of "num_iter(tag in wannier 90)" is higher, spread of Wannier function (=WF) is gradually lower in wannier 90. If so, is this procedure that minimize the spread of ...
Y. S. Lym's user avatar
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Which experiments can offer insights about Hubbard $U$ parameter?

When considering $\mathrm{DFT}+U$ calculations, people either go with (1) first-principles approach: calculating the $U$ parameter using linear response theory, $\mathrm{DFPT}$, $\mathrm{ACBN0}$, etc.,...
Abdul Muhaymin -Free Palestine's user avatar
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Difference between GW gap and BSE gap. How does net charge play a role?

The energy gap from BSE (GW-BSE) is just the lowest optical excitation energy. The energy gap from GW, from what I read, is the electron affinity (energy of adding an electron). I suppose this means ...
Bohan Xu's user avatar
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Intuition about SCAN-DFT

I am currently performing molecular dynamics simulations of water/ice and would like some intuition behind some of the results I am seeing. For context, I am using the water model described in this ...
Baba Booey's user avatar
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Total ground state energy calculation of an electron-ion system using plane wave basis sets

Recently I have begun working on a project which involves constructing a "simple" density functional theory (DFT) code using a plane wave basis set in Python. My first step has been to try ...
m.roussev's user avatar
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Will the number of electron be conserved during Density functional theory self-consistent field calculation?

I am confused about the concept of charge neutrality in DFT calculations. Let's say I have two silicon atoms in a primitive unit cell. Since the silicon has 14 protons and 14 electrons in a charge ...
T.H. Kim's user avatar
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Can we use Density Functional Theory (DFT) to calculate the second and third-order force constants of a slab?

If we aim to compute the surface phonon lifetime of a Cu(111) surface, the second and third-order force constants are essential (to put in Boltzmann transport equation). However, to my knowledge, it ...
James's user avatar
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DFT total energy from band energy

I'm using Kohn-Sham DFT as a part of my research. The material is metallic crystal. In the following, you can assume that $\rho$ refers to the density matrix and $H$ refers to a hamiltonian matrix ...
Mikke Mus's user avatar
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Spin polarization due to exchange? Would spin polarized ground state exist if no $e$-$e$ repulsion?

Non-relativistic no-magnetic-field many electron hamiltonian contains no spin operators. How would spin polarization happen in many electron ground state (modeled by LSDA DFT for instance)? I often ...
Bohan Xu's user avatar
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Can topologically non-trivial edge states exist without an energy gap?

I am relatively new to the field of topological materials, and I came across a paper that claims that they found topologically non-trivial states in a material. Basically, the authors found a type-I ...
Mikhail Petrov's user avatar
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Is there an Optimized Norm-Conserving Vanderbilt (ONCV) pseudopotential of $\rm Eu$ atom?

I'm currently finding an Optimized Norm-Conserving Vanderbilt (ONCV) pseudopotential of $\rm Eu$ atom which has a localized $f$-orbital. But no matter how much I search in online, I can't find that. ...
Y. S. Lym's user avatar
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Can we use DFT to compute the potential energy of dynamic processes?

Suppose we want to determine the potential energy between a hydrogen atom and a surface, such as Cu(001), during hydrogen scattering on Cu(001) surface, assuming that the hydrogen atom does not change ...
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No of Nearest Neighbour Magnetic ions in a Magnetic unit cell of a Solid

Total energy for MnO magnetic solid by considering a spin configuration in which all Mn Spins are ferromagnetically aligned can be written as: by mapping the energy on following the Heisenberg model ...
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Occupation numbers under the independent electron assumption

I am working with DFT, so the assumption is that the many-body Hamiltonian describes many independent electrons. I have solved the eigen-system and received the energy levels of the electrons and the ...
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Density functional matrix for network analysis

I am an archeologist just beginning to venture into the study of crystalline materials, specifically ceramics, to understand how their spatial distribution and formation process can be studied using ...
Jayeeta Ghosh's user avatar
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Where to find model parameters of a simple Weyl semimetal?

A famous continuum model of Dirac semimetal is essentially two copies of the following Hamiltonian (up to some chirality reversal) $$h=\begin{pmatrix} M(k) & Ak_+ \\ Ak_- & -M(k) \end{pmatrix}$...
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Where is the kinetic energy functional error in the exchange-correlation functional of DFT?

The Kohn-Sham DFT energy functional is: $$E[\rho]=T^S[{\varphi_i}]+J[\rho]+M[\rho]+E^{xc}[\rho]$$ with the kinetic energy functional of non-interacting electrons $T^S$, the Hartree functional $J$, the ...
Guiste's user avatar
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In solid state physics and Density Functional Theory, how are Van Der Waals forces modelled?

Given a material, I'd like to know how to treat VdW interactions among layers. Specifically I'm using Quantum Espresso, an open-source suite based on Density Functional Theory, and I'd like to know ...
CoolerThanACooler's user avatar
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How to induce the Phonon Effective mode mass?

i don't know the phonon effective mode formula. Who knows why effective mode mass formula have the following form? https://doi.org/10.1103/PhysRevB.104.L060103 PDF
Y. S. Lym's user avatar
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Exchange correlation functional for harmonic oscillators

Is it possible to construct the exchange correlation functional in DFT for some exactly solvable system? My proposal is to look at a system of Harmonic oscillators. Consider two electrons moving in ...
phonon's user avatar
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Why do IR spectra of calculation and experiment differ?

Recently I was simulating the IR spectra ( 400-3500 $cm^{-1}$ ) of an aromatic-like molecule with DFT. Then I did a comparison to the available experimental data. Most data fitted well together: ...
Lockhart 's user avatar
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Electric field from DFT calculations in Vasp

This is a naive question. I am studying the surface-adsorbate systems using DFT calculations implemented in VASP. By setting LVHAR = True, I obtain LOCPOT which contains information about the ionic ...
Batman's user avatar
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Some questions about the derivation process of many-body Green's functions (specifically, the Hedin equations)

The response of the Green's function under the action of an external field can be represented by the density correlation function L: $$L_{s_1 s'_{1} , s_2 s_{2'} }(12) = - i \hbar \frac{ \delta G_{s_1 ...
Liang's user avatar
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Significance of Self-Interaction in Hartree Potential

According to DFT (and correct me if I'm incorrect) the terms of Hartree potential and exchange correlation potential are introduced to simulate the particle (fermion-fermion) interaction in our system ...
Harshdeep Chhabra's user avatar
2 votes
1 answer
162 views

Coulombic Interaction in Condensed Matter

I am reading introductory DFT (Density Functional Theory) and I came across this, Can someone explain why the system needs to be neutral for energy to be finite? and What does organising in neutral ...
Harshdeep Chhabra's user avatar
8 votes
3 answers
938 views

Density functional theory: logical steps from Hohenberg-Kohn theorems to Kohn-Sham equations

I'm trying to learn density functional theory (DFT), using Engel & Dreisler, Sholl & Steckel, and Wikipedia as 3 different sources of information. Although I have made some progress, I am ...
Kenny Wong's user avatar
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Rewriting two-body operator in second-quantized form

I would like to understand the following identity for fermion field operators: $$\psi^\dagger(x) \psi^\dagger(y) \psi(y) \psi(x) = \psi^\dagger(x) \psi(x) \psi^\dagger(y) \psi(y) - \delta(x - y) \psi^\...
DJMeister's user avatar
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How to derive the following equation in DFT?

Recently, I have read a paper (J.Chem.Phys,99,1993 by Kais and others: download here) which uses Harmonium model to inverse Kohn-Sham equation and obtain exchange-correlation potential. Without going ...
Wisdom's user avatar
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I'm trying to solve the expression for Dirac exchange, but I'm not understanding the variables change employed. How can I perform it? [closed]

I'm trying to perform the variables change employed in the book "Density Functional Theory of Atoms and Molecules", by Parr and Weitao - page 108. But, from the expression of \rho_{1} given ...
lphys's user avatar
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Gibbs energy calculations for compounds

I am studying machine learning during my master's degree and have a simple task. I am using the FactSage Pure Substance Database (https://www.crct.polymtl.ca), which provides a list of phases for a ...
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Hartree-Fock exchange energy: can $i = j$?

I am trying to understand the Hartree-Fock exchange energy used in a paper by Becke and Roussel [1], where it's defined as: $$\begin{align} E_X &= \sum\limits_{\sigma} E_{X\sigma} \tag{1}\\ &= ...
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Commutator in the derivation of the Runge-Gross Theorem

In the derivation of the Runge-Gross theorem, a theorem that states a one-to-one correspondence between potential and density in an evolving system, a commutator appears that I seem to have trouble ...
Jannis Erhard's user avatar
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Germanium bandgap with DFT+U calculations

DFT calculations with GGA functionals result in bulk germanium being metallic. Has anybody ever tried to correct this result by computing the bandgap with DFT+U in Quantum ESPRESSO? I tried but failed....
mrf1g12's user avatar
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Atoms or molecules as strongly correlated systems

Electrons in atoms or molecules are of course correlated, in the sense that the many-electron wave function is not a Slater determinant. However, in my personal impression, the Hartree-Fock ...
poisson's user avatar
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How is the band gap really defined and calculated?

When I look into literature I am quite confused on how the band gap of a semiconductor is defined. One statement I often read goes something like that: The band gap of a semiconductor is the energy ...
Lockhart 's user avatar
3 votes
1 answer
282 views

Is Thomas-Fermi screening the result of a Thomas-Fermi model in an external field?

On the one hand, the theory of Thomas-Fermi screening describes the response of an ideal metal (or electron gas) in the presence of an external field. In this theory, one assumes that you have a Fermi ...
Mauricio's user avatar
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Hohenberg-Kohn Hamiltonian Expression

In Hohenberg and Kohn's paper on the inhomogeneous electron gas they express the Hamiltonian for "a collection of an arbitrary number of electrons moving under the influence of an external ...
gigo318's user avatar
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Do different Hamiltonians result in different ground states?

I'm learning density functional theory. In the proof of Hohenberg–Kohn theorem I, we assume that different Hamiltonians result in different ground states. Is it true? In general, for example, we can ...
jiawei chen's user avatar
4 votes
2 answers
232 views

From molecular orbitals to band diagram

Let’s say we have a periodic crystal structure. We could, in theory, treat this system as if it were a large molecule. Therefore, we could use Hartree Fock theory or other methods to get the molecular ...
Lockhart 's user avatar
1 vote
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Why is the commensurate charge density wave for 1T-$\rm TaSe_2$ based on non-integer dimensions?

I have read several papers that have stated that the commensurate charge density wave (CDW) for 1T-$\rm TaSe_2$ comes from a $ \sqrt{13}a_0$ x $\sqrt{13}a_0$ periodic lattice distortion where $a_0$ ...
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Why is there no exchange interaction when there is no coulomb interaction?

The Hartree-Fock energy may be written as $$ E_{HF}= \langle\Psi| H |\Psi\rangle = \sum_{a} \langle a| h |a \rangle + \frac{1}{2}\sum_{ab} \big( [aa |bb] - [ab|ba] \big),$$ with the exchange term $$[...
Lockhart 's user avatar
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DFT Hartree Potential with Periodic Boundary Conditions

Suppose we have a periodic crystal. Let $\rho(r)$ be the electronic density, and let $a$ be the lattice vectors. Due to the periodicity, the Hartree potential can be written as \begin{align} V_H(r)&...
Calvin Ku's user avatar
3 votes
1 answer
224 views

Mixed second functional derivative not symmetrical with respect to order of differentiation

I have a functional $E$, which is a functional of two different functions and their gradients: $$ E[\psi_1,\psi_2] = \int d^3\mathbf{x} ~ \varepsilon\{\mathbf{x}\}$$ where I'm using $\varepsilon\{\...
Jospeh's user avatar
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When do we need to consider Quantum electrodynamic density functional theory?

We know well quantum electrodynamics (QED) and density functional theory (DFT). The former describes the quantum nature of electromagnetic field, and the latter typically concerns the quantum and many-...
Pu Zhang's user avatar
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What is the mechanism behind the spectral property change in voltage sensitive dyes?

As far as I understand the change in spectral properties of voltage sensitive dyes is partially derived from the Stark Effect. However, even if this is the underlying physical phenomena, it is still ...
Evamentality's user avatar
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How to derive the expression for the effective potential in Kohn-Sham operator in Kohn-Sham density functional theory?

In density functional theory the Kohn-Sham method provides a systematic way to approaching the correct electron density of a given system. Kohn-Sham method uses a non-interacting reference system ...
Programmer's user avatar
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141 views

How to calculate the orbital character for fatbands in Quantum Espresso?

we are calculating a fatbands structure using QE pw.x. When We get the projections by weight. There are orbitals missing in the projection file. Is there a way to get contribution about outermost ...
Pamela Martínez's user avatar
4 votes
1 answer
5k views

Hartree-Fock vs. density functional theory

What is the relationship/difference between the Hartree-Fock method and the density functional theory? It seems the basic formulations of them are very similar. Which one is more accurate?
Hans's user avatar
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Understanding the derivation of the variational principle in classical density functional theory

I am trying to understand the derivation of the variational principle as presented in Bob Evan's 1979 work.[1]. The part that is tripping me up is when he presents the following result. He starts by ...
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