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21 votes

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

Short answer: Modern implementations of these two methods lead to similar accuracies. Longer answer: The calculation of phonons requires the calculation of the Hessian of the potential energy surface ...
ProfM's user avatar
  • 11.1k
14 votes

Reliability about the predicted two-dimensional crystal structures based on DFT high-throughput calculation?

[Disclaimer - I am one of the co-authors of the 2D database on Materials Cloud (What you call "2D structures and layered materials", publishing the data of this work: N. Mounet et al., ...
Giovanni Pizzi's user avatar
10 votes

Why do we need to build a supercell to use the DFPT in Phonopy interfaced with VASP?

Disclaimer: I have never used Phonopy. The advantage of using DFPT is that in principle it can be used to calculate a perturbation of finite wave vector $\mathbf{q}$ using the primitive cell. This ...
ProfM's user avatar
  • 11.1k
9 votes

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

This is actually an important question regardless of the code you're using (I use SIESTA and NWChem). To obtain a good phonon spectrum, the first thing you need is a truly relaxed/optimized structure. ...
Elie H's user avatar
  • 1,411
7 votes

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 ...
Jaafar Mehrez's user avatar
6 votes

What is the importance of electron interaction on dielectric response of crystals?

I think you should take care of all possible interactions to get close to the real picture. In periodic solids, there might be electron-hole interaction (solve BSE equation for it), el-phonon coupling,...
Haseeb Ahmad's user avatar
5 votes

DFPT vs analytical nuclear gradient

These are not the same quantity. The gradient of the energy with respect to the position $\vec{R}_i=(x_i,y_i,z_i)$ of atom $i$ gives the force, $\vec{F}_i$, acting on that atom, $$ \vec{F}_i=-\left(\...
Phil Hasnip's user avatar
  • 8,142
4 votes

Do the bottom layers of the slab need to be fixed when we use DFPT or finite displacement methods to compute the phonon spectrum of a surface?

No you do not need to. Fixing atoms during a finite displacement or a DFPT calculation is not a real thing. This is a huge approximation that will break the symmetry of the force constant matrix, ...
Okano's user avatar
  • 1,417
4 votes

First order variation of the wave function of conduction states

Let's start by unpacking Eq. (1), which we rewrite here: $$\begin{equation} (H_{SCF} - \varepsilon_n)\lvert \Delta \psi_n \rangle = -(\Delta V_{SCF} + \Delta \varepsilon_n) \lvert \psi_n \rangle. \tag{...
elutionary's user avatar
  • 1,006
2 votes

Phonon frequencies and free energy with respect to polymorphs

Your interpretation of the results. I agree with you that if you find no imaginary frequencies in the cubic phase it means it is at a local minimum of the potential energy landscape, and that if you ...
ProfM's user avatar
  • 11.1k
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 ...
Tyler Sterling's user avatar

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