# Questions tagged [phonons]

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

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59 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 ...
72 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? (...
728 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 ...
91 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
95 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 ...
938 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 ...
152 views

### Phonon density of states as a function of frequency vs energy

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 ...
1k 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 ...
57 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\...
90 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?
245 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 ...
173 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-...
109 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 ...
927 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 ...
188 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?
118 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 ...
120 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 ...
595 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 ...
140 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?
57 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}{...
73 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 ...
70 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 ...
534 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 ...
93 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?
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 ...
310 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 ...
222 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.
103 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 ...
108 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 ...
212 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.