Questions tagged [phonons]

Questions related to phonons as applicable to the field of materials science.

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K path for phonon band structure

I have observed in research papers that the K path chosen for electronic band structure and for phonon band structure to be different. I am currently using VASPKIT in order to obtain the path for ...
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5 votes
0 answers
45 views

Stability using Phonon Calculation

While performing phonon calculation using finite difference method, we get normal modes printed in the outcar file. Now there is a distinction between two different types of modes in the outcar file. ...
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2 votes
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How to determine the TA/LA by phonopy

I would like to ask how to use PHONOPY to define which band is LO, TO or ...
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4 votes
0 answers
33 views

Phonon branch-resolved harmonic sampling

I would like to do a phonon band-resolved harmonic sampling, following the recipes from the paper by West and Estreicher (harmonic sampling). In fact, the method presented in the paper is already ...
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9 votes
2 answers
108 views

How to calculate temperature dependent phonon band structure in VASP

I want to calculate phonon band structure using VASP for a double perovskite $\ce{A2BB'X6}$. Most of the perovskites are well known for dynamical instability at $\pu{0K}$ but show a dynamically stable ...
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5 votes
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42 views

How to derive specific heat of a crystalline material from phonon density of states?

I have done a simulation of a crystalline material using DFT and have extracted its normal modes and its phonon density of states. Does anyone have an algorithm/code or a detailed resource that can ...
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4 votes
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34 views

How does the VIBRA module of SIESTA compute IR intensities?

I am working with Fluoro-Graphene (let's assume a fully relaxed monolayer on the XY plane with 16C-1F cell where the F is "covalently" bound to the graphene on top of it with respect to the ...
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  • 1,256
10 votes
1 answer
297 views

Tools to Visualize Phonons

Software such as ChemCraft or GaussView is easy to find for vibrational calculations in gas phase, but I am finding it much more difficult to find good visualization software for phonons. I have ...
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9 votes
1 answer
88 views

How to improve a phonon spectrum calculation for a monolayer material?

I am a VASP user, and I would like to know what are the things to keep in mind to obtain a good phonons spectrum curve without imaginary frequencies in terms of: Which method is better for monolayers?...
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7 votes
1 answer
258 views

How to solve this Phonopy-related problem?

I have used the finite displacements method for phonon spectrum calculation, and when I typed the command to create force sets: ...
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  • 5,511
7 votes
1 answer
157 views

Which IBRION tag should be used for finite displacement method in calculating Phonons spectrum?

I am planning to use the finite displacement method for calculating phonon spectrum, and I have prepared the INCAR file below to try it on a transition metal trihalide monolayer (e.g., VI$_3$). I ...
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6 votes
1 answer
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Which one should be investigated first? dynamic stability or magnetic stability?

I would like to investigate the stability of $\ce{VI_3}$ monolayer, but I am confused about whether I start directly by performing Phonon spectrum calculation, or I should first investigate the ...
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6 votes
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What is the minimum supercell size for phonon spectrum calculation for this monolayer system?

Following the previous threads on this topic, I would like to know the minimum supercell size suitable for Phonon spectrum calculation using Phonopy (DFPT) for CrI$_3$ monolayer system given by this ...
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10 votes
1 answer
140 views

Can electron-phonon coupling be extracted from the dielectric function?

Computationally, there are various ways to calculate the electron-phonon coupling constants as a function of phonon index and momentum. My question is about the reverse process. Let us say that an ...
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10 votes
1 answer
179 views

Quantum ESPRESSO ph.x output

I wanted to calculate the normal modes of some particular material using the Quantum ESPRESSO. Everything went fairly well, but there is a couple of lines in the output that I do not understand. In ...
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6 votes
0 answers
241 views

Error while installing ShengBTE

I am trying to install the thirdorder script that is mean to work in tandem with ShengBTE on an Ubuntu OS, but I'm getting the following error: ...
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5 votes
1 answer
157 views

Supercell model for a disordered system in phonon calculations using the harmonic approximation

I studied a disordered state (FCC solid solution) and used a special quasirandom structure supercell with 32 atoms to represent the system. After relaxing it thoroughly with ...
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8 votes
1 answer
117 views

Phonon frequencies and free energy with respect to polymorphs

I have a material that can crystallize in either a cubic or tetragonal polymorph depending on the synthesis. I used DFPT in VASP and calculated the phonon band structures. I see that the cubic has all ...
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7 votes
1 answer
101 views

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

Lattice dynamics codes like phonopy output vibrational frequencies at q-points. Can we get data of vibrational frequencies for each of the atoms in the supercell? (...
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7 votes
5 answers
2k views

Quantum harmonic oscillator, zero-point energy, and the quantum number n

The energy of a quantum harmonic oscillator is given as, \begin{equation} E_{qho} = \left(n + \frac{1}{2}\right)\hbar\omega,\;\;\;\;\;\; n = 0,1,2,3,... \tag{1} \end{equation} I understand the ...
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10 votes
1 answer
728 views

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

I would like to ask how to use PHONOPY to define which band is LO, TO or LA, TA, or ZO or ZA. I attach a picture from https://iopscience.iop.org/article/10.1088/0953-8984/28/10/103005
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9 votes
1 answer
176 views

Is there an upper limit of temperature after which the quasi-harmonic approximation (QHA) fails?

I can't seem to articulate what made me ask this question but it sprang up when I asked: What does it mean to say a material is 'anharmonic'?. I've heard before that the electronic entropy ...
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14 votes
2 answers
1k views

What does it mean to say a material is 'anharmonic'?

I understand that a harmonic picture of the potential in a material isn't enough to study lattice dynamics thoroughly. The quasi-harmonic approximation is a good workaround and helps incorporate ...
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10 votes
0 answers
193 views

Phonon density of states as a function of frequency vs energy [closed]

Lattice dynamics codes like Phonopy output density of states, $g_{(\omega)}$, as a function of frequency. Since $\int g_{(\omega)}d\omega$ is the number of states ...
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18 votes
1 answer
2k views

What do negative phonon frequencies signify?

Negative frequencies "in general" mean the direction of displacement is reverse to that in positive frequencies. Does it mean two frequencies with opposite signs but the same magnitude give ...
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6 votes
0 answers
102 views

Odd/even q-point mesh in harmonic phonon density of states (DoS) calculations [closed]

Total DoS for two different q-point grids, $100\times100\times100$ and $101\times101\times101$, are graphed above. The DoS for $63\times63\times63$, $65\times65\times65$, $71\times71\times71$, $72\...
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11 votes
1 answer
199 views

Phonon dispersion and phonon DOS

A follow up question for this question. What are the different properties that can be deduced from a phonon dispersion curve and Phonon DOS? Also if possible explain how to deduce them?
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10 votes
2 answers
339 views

Morse potential for phonons in solids instead of the harmonic potential approximation?

As the title says, the Morse potential (https://en.wikipedia.org/wiki/Morse_potential) is an exactly solvable model in quantum mechanics, just like the quantum harmonic oscillator. Extensions of it ...
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4 votes
1 answer
506 views

Best practice(s) for q-points convergence in phonon density of states calculations

The answer can be a complete description of all the things we must be attentive to while converging q-points. A peculiar thing that I feel contradicted about: Generally, we don't take an equal # of k-...
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8 votes
1 answer
303 views

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

For electronic density of states (DoS) calculations with VASP, it is recommended that VASP's internal symmetry routine is turned ...
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18 votes
2 answers
950 views

Logic of published geometry optimization results without checking phonons

I see that in many papers where optimization results presented, they do not mention phonons and their force convergence criterion is 0.01 eV/A. Can you publish a result like that? And is not this ...
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7 votes
1 answer
701 views

DFPT vs Finite Difference Method [duplicate]

Which one is better in case of a phonon calculation and does DFPT also have small negative frequencies near Gamma due to the numerical noise? What are their advantages and disadvantages?
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13 votes
1 answer
202 views

What is a "Charge Density Wave" and how is it related to the imaginary frequencies on phonon dispersion bandstructures?

I see that for some structures that have imaginary frequencies, they mention CDW, but I could not find any reasonable explanation about the relation between them. Does CDW cause instability in the ...
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  • 791
9 votes
2 answers
297 views

"Input forces are not enough to calculate force constants" Phonopy error?

I am doing a phonon calculation for a phase using a 2x2x1 supercell (after geometry optimization of the structure). I am getting an error when computing the DOS and band structure after computing the ...
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13 votes
2 answers
629 views

What does it mean if a strictly optimized experimental geometry presents imaginary frequencies?

Let's say you optimize an experimental geometry of a structure with very large basis sets, strict optimization settings with different XC functionals and after calculating phonons of this structure ...
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10 votes
2 answers
285 views

What are the methods that can be used to ensure that an optimized geometry is a local minimum?

Usually it is suggested to do a phonon calculation. What are the other ways?
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  • 1,859
7 votes
1 answer
92 views

Obtaining phonon density of states from Density Functional Theory

From Statistical Mechanics obtain an expression for the heat capacity of a solid is given by1: \begin{equation} \tag{1} {C_V}\left( T \right) = k\int\limits_0^\infty {\frac{{{{\left( {uv} \right)}^2}{...
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  • 19k
9 votes
1 answer
159 views

Phonon Calculations and Vibrational Modes

The Raman spectrum of a material shows certain 'active' modes for which we get experimental signatures. Phonon calculations are mostly used to calculate vibrational modes. How can we determine the ...
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11 votes
1 answer
100 views

Phonon Calculations for Bilayer 2D systems

I am facing problems in calculating the phonon band structure for bilayer 2D systems. While the band structure mostly shows positive frequencies for the 1 layer system, going to 2 layers makes some ...
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17 votes
2 answers
756 views

How to deduce phase transitions from a phonon calculation?

I came across the concept of using phonons to establish a material's dynamical stability, based on whether or not imaginary frequencies are present in its phonon band structure. What I am struggling ...
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4 votes
0 answers
116 views

What is the Kohn anomaly? [closed]

I have been unable to figure out what the Kohn anomaly is. What exactly is it, and why is it important?
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  • 1,859
26 votes
3 answers
2k 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 ...
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  • 1,859
16 votes
1 answer
529 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 ...
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14 votes
1 answer
263 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.
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  • 1,859
13 votes
2 answers
191 views

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

I realize that the more ordered the supercell is, the lower the number of displacements. Perhaps, we could say the same with respect to symmetry. Is there anything that can be done to reduce the ...
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17 votes
2 answers
145 views

How to simulate atomic scattering from solid walls at finite temperature for a particles in a box simulation?

I'm not sure if this will be considered on-topic, let's see what happens. I've asked this previously in Physics SE and it was closed as needing focus. It is now past 30 days and too late to consider ...
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16 votes
2 answers
678 views

Computing phonon dispersion from molecular dynamics simulations?

How would one obtain the phonon dispersion curve from a molecular dynamics simulation trajectory? What are the steps involved? Are there any packages that does this? If so, please mention them.
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26 votes
1 answer
192 views

How to map molecular vibrations into lattice phonons?

Let's assume I have already relaxed a molecular structure for a non-trivial molecule and obtained a geometry that is close to the one resulting from X-ray crystallography. This can be achieved in a ...
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  • 1,421
18 votes
1 answer
1k views

Are there differences in accuracy and reliability between the frozen phonon method and Density Functional Perturbation Theory?

When proposing or predicting new materials, whether in the bulk or in two dimensions, it is important to discuss the stability of the system. This is usually done through thermodynamics and(or) ...
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