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What do negative phonon frequencies signify?

Phonons are a measure of the curvature of the potential energy surface about a stationary point. In particular, the matrix of force constants is calculated as: $$ D_{i\alpha,i^{\prime}\alpha^{\prime}}(...
ProfM's user avatar
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12 votes
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Phonon dispersion and phonon DOS

The phonon dispersion relates the phonon frequencies $\omega_{\mathbf{q}\nu}$ for each branch $\nu$ with the phonon wave vector $\mathbf{q}$, typically along a path in the Brillouin zone joining high-...
ProfM's user avatar
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6 votes
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VASPKIT and SeeK-path recommend different paths. Which one to choose?

If you check out this picture of the Brillouin zone of a hexagonal cell, you can see points like L, A and H are defined for a 3D Brillouin zone. This is because the k-path finding algorithms are ...
CW Tan's user avatar
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6 votes

Obtaining phonon density of states from Density Functional Theory

Short answer. Yes, it is possible to calculate the phonon density of states using density functional theory. You can calculate the phonon frequencies on an arbitrarily large $\mathbf{q}$-point grid to ...
ProfM's user avatar
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5 votes

Phonon density of states in statistical thermodynamics

There are two possible things tripping you up here: Phonons are collective oscillations: they involve the motion of all the atoms together. Therefore it only makes sense to talk about the phonons of ...
taciteloquence's user avatar
4 votes

Should all q-points in the first Brillouin zone be sampled in phonon density of states calculations?

Background. The phonon density of states $g$ is given by: $$ \tag{1} g(\omega)=\sum_{\nu}\int\frac{d\mathbf{q}}{(2\pi)^3}\delta(\omega-\omega_{\mathbf{q}\nu})\approx\frac{1}{N_{\mathbf{q}}}\sum_{\nu}\...
ProfM's user avatar
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4 votes

Causes of Straight Bands in Phonon Dispersion Calculations

Such bands are usually called 'flat bands'. The group velocity vanishes, signalling that such an excitation is non-dispersive, i.e. is localized. See here. Think 'molecular modes'. For an isolated ...
Tyler Sterling's user avatar
3 votes

Can we translate vibrational frequencies at q-points to vibrational frequencies of atoms?

As known [1], the eigenvalues of the dynamical Hessian matrix represent the phonon frequencies, whereas the eigenvectors represent the particular atomic displacement patterns contributing to the ...
Evgeny Blokhin's user avatar
3 votes

Exploring Phonon Dispersion in Two-Dimensional Materials: Analytical Techniques and Mode Identification

Do you know what longitudinal, transverse, acoustic, and optical mean in this context? Or you want a code that can tell you what they are? If you know what they mean, you can plot the displacements ...
Tyler Sterling's user avatar
2 votes

project molecular dynamics trajectory onto phonons by normal mode decomposition

DynaPhoPhy DynaPhoPhy is an open source (MIT License) python library for Phonon anharmonic properties from Molecular Dynamics. DynaPhoPhy allows to calculate anharmonic phonon linewidhts and ...
Vandan Revanur's user avatar
1 vote

Negative phonon frequencies at finite temperature

If you perform an atom relaxation before the phonon, you also get a 0 K result wrt. atom positions. You will need to perform an MD simulation to simulate effects from temperature properly. I would ...
Tristan Maxson's user avatar

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