# phonons

- 7.1 Is there a simple way to determine the symmetry of a given phonon mode?
- 7.2 I am not getting zero acoustic mode frequencies, why?
- 7.3 Why do I get negative phonon frequencies?
- 7.4 Why do I get a message
*no elec. field with metals*? - 7.5 Mysterious symmetry-related errors
- 7.6 How can I calculate Raman/IR coefficients in metals?
- 7.7 How can I calculate the electron-phonon coefficients in insulators?
- 7.8 What is the format of the file containing interatomic force constants?

### 7.1 IS THERE A SIMPLE WAY TO DETERMINE THE SYMMETRY OF A GIVEN PHONON MODE?

A symmetry analyzer was added in v.3.2 by Andrea Dal Corso. Other packages that perform symmetry analysis of phonons and normal modes:

ISOTROPY package: http://stokes.byu.edu/iso/isotropy.html

ACKJ, ACMI packages: http://www.cpc.cs.qub.ac.uk.

### 7.2 I AM NOT GETTING ZERO ACOUSTIC MODE FREQUENCIES, WHY?

Because the Acoustic Sum Rule (ASR), i.e. the translational invariance, is violated in approximated calculations. In PW calculations, the main and most irreducible violation comes from the discreteness of the FFT grid. There may be other reasons, though, notably insufficient convergence: “Recently I found that the parameters tr2_ph for the phonons and conv_thr for the ground state can affect the quality of the phonon calculation, especially the “vanishing” frequencies for molecules.” (Info from Katalyn Gaal-Nagy). Anyway: if the nonzero frequencies are small, you can impose the ASR to the dynamical matrix, usually with excellent results.

Nonzero frequencies for rotational modes of a molecule are a fictitious effect of the finite supercell size, or else, of a less than perfect convergence of the geometry of the molecule.

### 7.3 WHY DO I GET NEGATIVE PHONON FREQUENCIES?

“Negative” frequencies actually are “imaginary” frequencies (\omega^2 \le 0). If these occur for acoustic frequencies at Gamma point, or for rotational modes of a molecule, see above. In all other cases: it depends. It may be a problem of bad convergence (see above), or it may signal a real instability.

### 7.4 WHY DO I GET A MESSAGE _NO ELEC. FIELD WITH METALS_?

If you want to calculate the contribution of macroscopic electric fields to phonons – a quantity that is well-defined in insulators only — you cannot use smearing in the scf calculation, or else the code will complain.

### 7.5 MYSTERIOUS SYMMETRY-RELATED ERRORS

Symmetry-related errors like *symmetry operation is non orthogonal, or wrong representation*, or *wrong degeneracy*, are almost invariably a consequence of atomic positions that are close to, but not sufficiently close to, symmetry positions. If such errors occur, set the Bravais lattice using the correct ibrav value (i.e., don’t use ibrav=0), use Wyckoff positions if known. This must be done in the self-consistent calculation.

### 7.6 HOW CAN I CALCULATE RAMAN/IR COEFFICIENTS IN METALS?

You cannot: they are well defined only for insulators.

### 7.7 HOW CAN I CALCULATE THE ELECTRON-PHONON COEFFICIENTS IN INSULATORS?

You cannot: the current implementation is for metals only.