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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →If a Quantum ESPRESSO pw.x self-consistent-field (SCF) calculation is slow, oscillates, or stops converging, check the structure and electronic setup before changing mixing parameters. Then match a targeted adjustment to the symptom: metallic occupations, charge sloshing, an ultrasoft-pseudopotential density issue, or an eigensolver failure each calls for a different response. No single setting guarantees convergence for every material.
Start by checking the input and model
Before tuning the SCF loop, check whether it is solving the problem you intended. Quantum ESPRESSO’s troubleshooting guide warns that bad input can cause poor convergence and specifically advises checking the structure.
- Inspect atomic positions and cell dimensions for malformed or chemically implausible geometry.
- Verify species labels and their pseudopotential assignments.
- Check the electron count, number of bands (
nbnd), and k-point sampling. - Review the relevant
&SYSTEMand&ELECTRONSsettings, including occupations and mixing.
Mixing changes cannot reliably compensate for a defective structure, incorrect electron count, or unsuitable calculation setup.
Check whether metallic occupations are appropriate
Metallic or near-metallic systems can behave differently from insulators in the SCF loop. The troubleshooting guide describes an oscillation in which the self-consistency error falls and then rises as the highest occupied and lowest unoccupied states exchange places. It suggests adding some empty bands and a small broadening in this situation.
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Choose occupations for the calculation
QE’s guide says occupations='fixed' works only for insulators with a gap, and recommends occupations='smearing' otherwise. It identifies occupations='tetrahedra' for density-of-states calculations. Choose based on the system and purpose of the run rather than applying one occupation setting indiscriminately.
Consider k-points, bands, and broadening together
If the system is metallic or close to metallic, inspect whether the k-point mesh is sparse and whether there are enough bands above the occupied states. A small broadening and additional empty bands may help with the occupation switching described in the guide. These are diagnostic adjustments, not guaranteed fixes.
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Reduce charge-density oscillation with mixing adjustments
If the input and occupation setup look sound but the density oscillates or convergence is simply slow, try a smaller mixing_beta. The official troubleshooting guide and self-consistency FAQ suggest about 0.3 to 0.1 or smaller as a starting range. This is not a universal optimum: change one setting at a time and compare the convergence history.
Choose a mixing mode that fits the density
The current pw.x input reference, for version 7.5, describes these mixing_mode choices:
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plain: charge-density Broyden mixing.TF: simple Thomas–Fermi screening for highly homogeneous systems.local-TF: local-density-dependent screening for highly inhomogeneous systems.
For slabs or elongated cells, the troubleshooting guide says local-TF may damp charge sloshing better. That makes it a geometry-specific option to compare, not a general replacement for other modes.
Adjust mixing history with its memory cost in mind
The input reference lists mixing_ndim with a default of 8; it is the number of iterations used by the mixing scheme. The troubleshooting guide says increasing it beyond 8 may help but costs memory. The input reference says it may be lowered to around 4 when memory is tight. Treat this as a resource trade-off, not a free speedup.
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Investigate the documented ultrasoft-pseudopotential density issue
For a specific ultrasoft pseudopotential (USPP) problem, QE’s troubleshooting guide describes negative charge-density regions associated with augmentation pseudization or truncation at a finite cutoff. It says raising ecutrho will usually help in that case. This recommendation is tied to that density and pseudopotential issue; it does not establish that ecutrho causes every SCF failure.
Separate eigensolver trouble from SCF mixing trouble
Changing the diagonalization method is relevant when there is evidence of an eigensolver problem, not as the default response to charge-density oscillation. The current input reference describes Davidson as fast but occasionally prone to failure, and conjugate-gradient diagonalization as slower, lower-memory, and a little more robust.
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diagonalization option |
Documented behavior | When to consider it |
|---|---|---|
'david' (default) |
Davidson iterative diagonalization with overlap matrix; fast, may rarely fail. | Default choice unless there is evidence of diagonalization trouble. |
'cg' |
Conjugate-gradient method; much slower, uses less memory, and is a little more robust. | When diagonalization reliability or memory use is the concern. |
Do not confuse the inner diagonalization threshold with the SCF energy-error threshold. The reference lists diago_thr_init defaults of 1.D-2 when starting from a superposition of atomic orbitals and 1.D-5 when starting from a charge density in SCF calculations; it says the threshold tightens automatically as self-consistency approaches convergence, never below 1.D-13. By contrast, conv_thr concerns estimated energy error and is extensive.
Treat cannot bracket Ef as a distinct diagnosis
The error cannot bracket Ef is not simply another sign that mixing needs adjustment. The troubleshooting guide lists serious input-data problems such as an incorrect electron count, too few bands, or absurd broadening among possible causes. Check those first.
The guide also notes that first-order Methfessel–Paxton smearing can cause difficulty with very few k-points because its integrated density of states is not guaranteed to increase monotonically. Gaussian broadening or Marzari–Vanderbilt–DeVita–Payne (cold) smearing are suggested alternatives in that situation.
Distinguish the band-structure case
For a calculation of selected high-symmetry lines, the same message can mean occupations and Fermi energy are incorrect even though eigenvalues and eigenvectors are valid. In this specific case, the guide says removing occupations='tetrahedra' removes the message. Do not treat this special band-structure condition as proof that a general SCF cycle has converged.
Match the next change to the observed failure
| Observed symptom | Compare or check |
|---|---|
| Occupation instability or metallic character | Occupation method, empty-band count, broadening, and k-point sampling. |
| Oscillatory density or charge sloshing | mixing_beta, mixing_mode, and possibly mixing_ndim, accounting for memory. |
| Slab or elongated geometry | Whether local-TF is suitable for damping charge sloshing. |
| USPP density behavior | Whether the documented charge-density/cutoff issue applies and whether ecutrho merits investigation. |
| Eigensolver failure or memory constraint | Davidson versus conjugate gradient, weighing speed, robustness, and memory. |
Change one relevant factor at a time and compare the run’s convergence history. The official guidance provides parameter suggestions and defaults, not a benchmark across materials or a setting that is best for every calculation.
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