Questions tagged [phonons]
Questions related to phonons as applicable to the field of materials science.
59
questions
3
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0
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40
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Kohn anomaly and avoided crossing (or anticrossing/level repulsion)
Kohn Anomaly and Avoided Crossing (Anticrossing/Level Repulsion) are terms that are found to be used when discussing phonon dispersion. The former (see last paragraph, page 198) is related to electron-...
- 375
6
votes
1
answer
50
views
Confusion regarding specifying a k-point path for phonon dispersions/bandstructure plots
I have been trying to obtain a plot of the phonon dispersion of a BCC metal using Quantum Espresso. In the literature it seems that a commonly reported path is ...
- 111
8
votes
1
answer
90
views
Zero point correction and free energy calculation with imaginary modes?
I am a beginner in DFT. Recently I have been working on calculating the energy (with zero point correction), and may be free energy in the future, for a structure in both stable and unstable ...
4
votes
0
answers
60
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An issue of running Phonopy for graphene using Siesta
I have used Siesta in the example attached with phonopy to run phonon for graphene, and when I typed the command to create force ...
- 41
2
votes
0
answers
28
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Phonon Angular Momentum
Can anyone help me understand what is Phonon Angular Momentum? How does it affect thermal properties? Any relevant books and papers are welcome, if you have suggestions.
- 1,015
5
votes
1
answer
97
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Can a material have negative phonon modes in the phonon spectrum but still exist?
I came across a paper which talks about $Cs_2SnI_6$ the material is well researched and there are dozens of experimental papers which say that the material exists in Fm3m space group(FCC), the ...
- 629
10
votes
1
answer
313
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Should one perform geometry optimisation before calculating electronic/phonon band structures via DFT?
I am new to DFT calculations and I would like to get the electronic and phonon band structures of FeSn (1 kagome layer and 1 Sn layer in between). My question is, is it absolutely necessary to first ...
- 101
3
votes
1
answer
113
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Simple and quick way to get phonon dispersion?
I want a simple phonon dispersion calculation, the lighter the program the better I do not want to install some huge package like QuantumExpresso just for that. Another way would be to find a database ...
- 332
4
votes
0
answers
30
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q-points weighting factor in QE
When using Quantum Espresso to perform phonon calculations, one can specify a grid of q-point using ldisp=.true. (Monkhorst-Pack grid)
If I ask for a AxBxC grid, the code will reduce this grid taking ...
- 1,098
7
votes
2
answers
226
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Details about ZPE and entropy of adsorbates
I want to compute the ZPE and entropic terms for my platinum slab covered by a bilayer of water (network). I have performed a phonon calculation on a 3x3 q-point grid (my unit cell is ~ 4 Å long)
The ...
- 1,098
5
votes
1
answer
109
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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 ...
- 1,641
9
votes
1
answer
197
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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.
...
- 111
2
votes
0
answers
62
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How to determine the TA/LA by phonopy [closed]
I would like to ask how to use PHONOPY to define which band is LO, TO or ...
- 21
5
votes
1
answer
59
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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 ...
- 71
10
votes
2
answers
325
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 ...
- 1,663
5
votes
0
answers
50
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How to derive specific heat of a crystalline material from phonon density of states? [closed]
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 ...
- 1,317
5
votes
0
answers
68
views
How does the VIBRA module of SIESTA compute IR intensities? [closed]
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 ...
- 1,317
10
votes
1
answer
740
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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 ...
- 10.6k
9
votes
1
answer
146
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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?...
- 5,593
7
votes
1
answer
530
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:
...
- 5,593
7
votes
1
answer
307
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 ...
- 5,593
6
votes
1
answer
57
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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 ...
- 5,593
6
votes
0
answers
139
views
What is the minimum supercell size for phonon spectrum calculation for this monolayer system? [closed]
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 ...
- 5,593
10
votes
1
answer
190
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 ...
- 303
10
votes
1
answer
285
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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 ...
- 335
7
votes
1
answer
367
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:
...
- 1,015
5
votes
1
answer
170
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 ...
- 5,065
8
votes
1
answer
139
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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 ...
- 1,050
7
votes
1
answer
118
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? (...
- 5,065
7
votes
5
answers
3k
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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 ...
- 5,065
10
votes
1
answer
1k
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
- 1,945
9
votes
1
answer
221
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 ...
- 5,065
15
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 ...
- 5,065
10
votes
0
answers
230
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 ...
- 5,065
19
votes
1
answer
3k
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 ...
- 5,065
6
votes
0
answers
130
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\...
- 5,065
11
votes
1
answer
391
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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?
- 8,612
10
votes
2
answers
386
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 ...
- 188
5
votes
1
answer
700
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-...
- 5,065
8
votes
1
answer
407
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 ...
- 5,065
18
votes
2
answers
961
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 ...
- 791
7
votes
1
answer
980
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?
- 791
13
votes
1
answer
260
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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 ...
- 791
9
votes
2
answers
410
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 ...
- 846
14
votes
2
answers
643
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 ...
- 1,919
10
votes
2
answers
426
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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?
- 1,919
7
votes
1
answer
164
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}{...
- 20.8k
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votes
1
answer
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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 ...
- 1,336
11
votes
1
answer
146
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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 ...
- 1,336
18
votes
2
answers
981
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 ...
- 283