next up previous contents
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 $\bf q$ = 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: 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 $\bf q$, the code compares $\bf q$ and the rotated $\bf q$, 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
next up previous contents
Next: A. Appendix: Electron-phonon coefficients Up: User's Guide for the Previous: 5 Parallelism   Contents