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How to determine LA/TA, ZA/ZO and LO/TO by PHONOPY

There are two major techniques used to model phonon interactions - The frozen phonon method and density functional perturbation theory (DFPT). Phonopy is used to carry out calculations in the frozen ...
• 2,422
Accepted

What do phonon dispersion (or lattice dynamics) studies include?

TLDR: When you calculate phonons, you can describe electrons at different levels of theory, typically semilocal DFT, but also hybrids or dynamical mean-field theory. Phonons do include zero-point ...
• 11.1k
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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-...
• 11.1k

Should one perform geometry optimisation before calculating electronic/phonon band structures via DFT?

For the electronic band structure, I'd say it's debatable. For consistency reasons, it would make sense for example to perform a geometry optimization with PBE and calculate its PBE band structure. ...
• 1,326

Phonon Calculations for Bilayer 2D systems

The presence of imaginary frequencies in a phonon dispersion can have two sources: Physical origin. If the imaginary frequency appears at a $\mathbf{q}$-point in the Brillouin zone that is included ...
• 11.1k

Phonon calculations using DFPT

WARNING (added by T.S., see comments below) As of 4/17/2024, VASP DFPT only works for $q=\Gamma$, thus you must use a supercell even with VASP-DFPT + Phonopy. Even the phonopy documentation uses a ...
• 3,500
Accepted

Can you calculate fat ("projected") phonon dispersion bands with Quantum Espresso?

These "projected phonon bandstructure" can be obtained using phonon eigenvectors. The coefficients (which determine the width of the lines on the plot) should just be the amplitude of the ...

Stability using Phonon Calculation

The basic quantity you build when performing a phonon calculation is the matrix of force constants, given by:  D_{i\alpha,i^{\prime}\alpha^{\prime}}(\mathbf{R}_p,\mathbf{R}_{p^{\prime}})=\frac{\...
• 11.1k

Can we do anything to reduce the number of displacements in a frozen phonon lattice dynamics study?

ProfM's answer gets the core idea perfectly right: Symmetry really is your best friend here. However, symmetry analysis is often quite involved, especially for larger unit cells. I recently ...
• 1,436

Can we do anything to reduce the number of displacements in a frozen phonon lattice dynamics study?

You can (should) use symmetry to reduce the number of displacements needed to construct the matrix of force constants. A nice pratical description of how to do this can be found in the description of ...
• 11.1k
Accepted

Are there other kinds of stability of materials? How to demonstrate that with computation approaches?

Radiation stability In some applications or environments (e.g. fission/fusion reactors, space, sterilization of packaging), radiation effects are highly important and can cause significant damage to, ...
• 4,761

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 ...
• 1,638
Accepted

Thermal vs dynamical stability

Let's see what is the difference between the two things that you are looking for: Dynamic Stability: This is usually calculated from ab-inito calculations, where you calculate the dynamic matrix for ...
• 4,037
Accepted

Getting the Phonon Dispersion Using fix phonon in LAMMPS

I was able to compile the phana code but fix phonon just gave a peak around zero. I raised an issue on the GitHub repository and ...
• 705

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 ...
• 1,638

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 ...
• 3,145
Accepted

Simple and quick way to get phonon dispersion?

The Materials Project Harnessing the power of supercomputing and state-of-the-art methods, the Materials Project provides open web-based access to computed information on known and predicted ...
• 23.7k

Why dipole correction does not work in VASP DFPT + phonopy calculation for a stable 2D material

I assume the problem has to do with the fact that the long-range Coulomb interaction looks different in 2D than it does in 3D. Dimensionality enters in Fourier space and yields a different asymptotic ...
• 136

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 ...
• 11.4k
1 vote

Inconsistencies in Phonon Dispersion Results Using Phonopy DFPT with Different Supercell Sizes

Based on your comments above, it looks like you increased k-point sampling for the 3x3x1 supercell and found the correct dispersions. It seems like the imaginary modes in the smaller cells is due to ...
• 1,638
1 vote

Simple Quantum Espresso-Phonon

ibrav=2 means that your lattice is fcc, i.e. if you put an Al atom at (0,0,0) , then three Al atoms are put at (1/2,1/2,0), (1/2,0,1/2), (0,1/2,1/2) automatically. Thus your input means that Al1 is at ...
1 vote
Accepted

Si Thin film phonon dispersion with negative frequency

Just because the structure is simple with high symmetry doesn't mean it's the correct ground state. More often that not, symmetry breaking leads to interesting physics (e.g. the surface reconstruction ...
• 1,638
1 vote

Question regarding q2r in QuantumEspresso Phonon Calculation

If you change the first file 'MoS2.dyn0' which should only contains information about the q-point grid to accommodate it to the fact you are missing some files things should work out. For example ...
• 1,427

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