Keywords: user_guide

Next: A. Appendix: Electron-phonon coefficients Up: User's Guide for the Previous: 5 Parallelism Contents

# 6 Troubleshooting

#### 6.0.0.1 ph.x stops with error reading file

The data file produced by `pw.x` is bad or incomplete or produced by an incompatible version of the code.

#### 6.0.0.2 ph.x mumbles something like cannot recover or error reading recover file

You have a bad restart file from a preceding failed execution. Remove all files `recover*` in `outdir`.

#### 6.0.0.3 ph.x says occupation numbers probably wrong and continues

You have a metallic or spin-polarized system but occupations are not set to ``smearing'`.

#### 6.0.0.4 ph.x does not yield acoustic modes with zero frequency at = 0

This may not be an error: the Acoustic Sum Rule (ASR) is never exactly verified, because the system is never exactly translationally invariant as it should be. The calculated frequency of the acoustic mode is typically less than 10 cm-1, but in some cases it may be much higher, up to 100 cm-1. The ultimate test is to diagonalize the dynamical matrix with program `dynmat.x`, imposing the ASR. If you obtain an acoustic mode with a much smaller ω (let us say < 1cm-1 ) with all other modes virtually unchanged, you can trust your results.

``The problem is [...] in the fact that the XC energy is computed in real space on a discrete grid and hence the total energy is invariant (...) only for translation in the FFT grid. Increasing the charge density cutoff increases the grid density thus making the integral more exact thus reducing the problem, unfortunately rather slowly...This problem is usually more severe for GGA than with LDA because the GGA functionals have functional forms that vary more strongly with the position; particularly so for isolated molecules or system with significant portions of ``vacuum'' because in the exponential tail of the charge density a) the finite cutoff (hence there is an effect due to cutoff) induces oscillations in rho and b) the reduced gradient is diverging.''(info by Stefano de Gironcoli, June 2008)

#### 6.0.0.5 ph.x yields really lousy phonons, with bad or ``negative'' frequencies or wrong symmetries or gross ASR violations

Possible reasons:

- if this happens only for acoustic modes at = 0 that should have ω = 0: Acoustic Sum Rule violation, see the item before this one.
- wrong data file read.
- wrong atomic masses given in input will yield wrong frequencies (but the content of file fildyn should be valid, since the force constants, not the dynamical matrix, are written to file).
- convergence threshold for either SCF (`conv_thr`) or phonon calculation (`tr2_ph`) too large: try to reduce them.
- maybe your system does have negative or strange phonon frequencies, with the approximations you used. A negative frequency signals a mechanical instability of the chosen structure. Check that the structure is reasonable, and check the following parameters:
  - The cutoff for wavefunctions, `ecutwfc`
  - For USPP and PAW: the cutoff for the charge density, `ecutrho`
  - The k-point grid, especially for metallic systems.
- For metallic systems: it has been observed that the convergence with respect to the k-point grid and smearing is very slow in presence of semicore states, and for phonon wave-vectors that are not commensurate i with the k-point grid (that is, ≠ - )

Note that ``negative'' frequencies are actually imaginary: the negative sign flags eigenvalues of the dynamical matrix for which ω2 < 0.

#### 6.0.0.6 Wrong degeneracy error in star_q

Verify the q-vector for which you are calculating phonons. In order to check whether a symmetry operation belongs to the small group of , the code compares and the rotated , with an acceptance tolerance of 10-5 (set in routine `PW/src/eqvect.f90`). You may run into trouble if your q-vector differs from a high-symmetry point by an amount in that order of magnitude.

#### 6.0.0.7 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. do not use `ibrav=0`), use Wyckoff positions if known. This must be done in the self-consistent calculation.

Subsections

- - - 6.0.0.1 ph.x stops with error reading file
    - 6.0.0.2 ph.x mumbles something like cannot recover or error reading recover file
    - 6.0.0.3 ph.x says occupation numbers probably wrong and continues
    - 6.0.0.4 ph.x does not yield acoustic modes with zero frequency at = 0
    - 6.0.0.5 ph.x yields really lousy phonons, with bad or ``negative'' frequencies or wrong symmetries or gross ASR violations
    - 6.0.0.6 Wrong degeneracy error in star_q
    - 6.0.0.7 Mysterious symmetry-related errors

Next: A. Appendix: Electron-phonon coefficients Up: User's Guide for the Previous: 5 Parallelism Contents