Questions tagged [phonons]
Questions related to phonons as applicable to the field of materials science.
32
questions
6
votes
0answers
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 ...
7
votes
1answer
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? (...
6
votes
5answers
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 ...
8
votes
1answer
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
7
votes
0answers
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 ...
14
votes
2answers
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 ...
9
votes
0answers
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 ...
15
votes
1answer
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 ...
6
votes
0answers
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\...
9
votes
1answer
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?
10
votes
2answers
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 ...
4
votes
1answer
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-...
8
votes
1answer
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 ...
17
votes
2answers
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 ...
7
votes
1answer
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?
13
votes
1answer
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 ...
9
votes
2answers
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 ...
13
votes
2answers
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 ...
10
votes
2answers
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?
6
votes
1answer
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}{...
8
votes
1answer
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 ...
9
votes
1answer
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 ...
16
votes
2answers
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 ...
4
votes
0answers
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?
25
votes
3answers
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 ...
15
votes
1answer
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 ...
14
votes
1answer
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.
11
votes
2answers
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 ...
17
votes
2answers
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 ...
12
votes
1answer
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.
26
votes
1answer
166 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 ...
16
votes
1answer
383 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) ...
