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17

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 $V(\mathbf{R})$, also known as the matrix of force constants: $$ \frac{\partial^2 V(\mathbf{R})}{\partial \mathbf{R}_i\partial\mathbf{R}_j}=-\frac{\partial \...


10

[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

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, etc. Note that, QE epsilon.x is the lowest level of approximation for the solids (IPA) and it doesn't include any non-local part and local field effects. ...


2

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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