# Tag Info

12

TLDR: When you calculate phonons, you can describe electrons at different levels of theory, typically semilocal DFT, but also hybrids or dynamical mean-field theory. Phonons do include zero-point motion, as they are essentially a set of uncoupled quantum harmonic oscillators. Enthalpy can be calculated without reference to phonons, simply adding a PV term to ...

10

The phonon dispersion relates the phonon frequencies $\omega_{\mathbf{q}\nu}$ for each branch $\nu$ with the phonon wave vector $\mathbf{q}$, typically along a path in the Brillouin zone joining high-symmetry points. The phonon density of states compresses this information by integrating over $\mathbf{q}$ and summing over $\nu$:  \tag{1} g(\omega)=\sum_{\...

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 ...

5

Radiation stability In some applications or environments (e.g. fission/fusion reactors, space, sterilization of packaging), radiation effects are highly important and can cause significant damage to, or changed properties of, materials. In other cases, radiation-induced changes of properties are in fact desired. For example, polymers are often irradiated to ...

5

ProfM's answer gets the core idea perfectly right: Symmetry really is your best friend here. However, symmetry analysis is often quite involved, especially for larger unit cells. I recently discovered the hiPhive package, which uses statistical fits (forces from random displacements fit to a force-constant potential), combined with with symmetry analysis (...

5

You can (should) use symmetry to reduce the number of displacements needed to construct the matrix of force constants. A nice pratical description of how to do this can be found in the description of the PHON package by Dario Alfè. In short: if you have the force constants for displacing a given atom, and when you apply the symmetry operations of the crystal ...

3

The presence of imaginary frequencies in a phonon dispersion can have two sources: Physical origin. If the imaginary frequency appears at a $\mathbf{q}$-point in the Brillouin zone that is included in the $\mathbf{q}$-point grid that you explicitly calculate (e.g. one of the points in the $N\times N\times N$ $\mathbf{q}$-point grid if you use a supercell of ...

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