# Tag Info

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[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., Nature Nanotech. 13, 246–252 (2018) so I will mostly refer to it below] In general, these studies "extract" a layer from a bulk 3D material, and then often ...

9

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 should be contrasted with finite differences, which can only be used to calculate perturbations at the $\Gamma$ point. If you want to access a non-$\Gamma$ wave ...

6

You can minimize the free energy F(a,c) as a function of (a,c), which is simply high school math. An example is provided in the section III (A) of the Supplementary Material of Phys. Rev. B 100, 161101(R) (2019): https://journals.aps.org/prb/supplemental/10.1103/PhysRevB.100.161101/supplemental.pdf

5

There are two major techniques used to model phonon interactions - The frozen phonon method and density functional perturbation theory (DFPT). Phonopy is used to carry out calculations in the frozen phonon scheme. ProfM does a fantastic job delineating the differences between the two methods here. The first step in such a finite displacement scheme is to ...

4

This error (input forces are not enough to calculate force constants) may arise when you are trying to plot the band structure or the density of state plots for some systems. Sometimes setting a higher symmetry tolerance for a structure may cause this type of error. A lower symmetry setting can solve the issue. The following command can be used in phonopy ...

4

It is always better to write your own code to get elastic constants. For example , In case of cubic system, we need three types of distorsion to unit cell. Now elastic constant is simply slope of second order fit of energies at different value of distorsion (see different publications) normalized with Volume minima (volume corresponds to minimum energy) . ...

3

Some years ago I found the following steps in ResearchGate: You need: QuantumEspresso (QE) Phono3py ShengBTE & thirdorder.py First, you need to create a QE input file (pwscf), using ibrav=0, lattice vectors in bohr units, atomic coordinates in crystal units. Using the input file, run phono3py/third_order.py to create supercell within finite ...

3

As known [1], the eigenvalues of the dynamical Hessian matrix represent the phonon frequencies, whereas the eigenvectors represent the particular atomic displacement patterns contributing to the vibrations. Therefore you might be interested in analyzing the eigenvectors and building the atomic visualizations based on them. In many cases (especially in the ...

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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 do find imaginary frequencies for the tetragonal phase, then that one is at a saddle point. I also agree that a phase exhibiting imaginary frequencies may be ...

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