The Tool Desk
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What do the cutoffs and k-point grid control?
In pw.x, ecutwfc sets the kinetic-energy cutoff for the plane-wave basis used to represent wavefunctions. ecutrho sets the cutoff for the charge density and potential. Both are expressed in Rydberg (Ry). Increasing a cutoff generally makes the basis representation more complete, at increased computational cost.
K_POINTS specifies sampling in reciprocal space. Its grid affects Brillouin-zone integrations and therefore can affect energies and other reported properties. Cutoff convergence and k-point convergence are separate tests: a result stable against one is not thereby proven stable against the other.
How do I choose ecutwfc in Quantum ESPRESSO?
Use the pseudopotential documentation or its recommended starting value to select an initial ecutwfc, then test higher values. A recommendation is a starting point, not evidence that your target result is converged. Keep the structure, pseudopotentials, functional, k-point grid, occupations, and other relevant inputs fixed while varying the cutoff.
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- Choose the quantity that matters for the calculation, such as total energy, forces, stress, or an electronic property.
- Run a sequence of calculations with progressively higher
ecutwfc, changing no other convergence parameter. - Compare the same quantity across the sequence. Stop only when further increases change it by less than the tolerance you have declared for the study.
- Record the values tested and the observed changes, along with the cutoff you selected.
There is no universal cutoff sequence or error tolerance: choose both for the pseudopotential, system, and accuracy needed for the reported result.
What should ecutrho be relative to ecutwfc?
In the Quantum ESPRESSO 7.5 input reference, ecutrho defaults to four times ecutwfc. The appropriate setting depends in part on pseudopotential family:
- Norm-conserving: QE advises keeping the default ratio. Reducing
ecutrhocan introduce noise, particularly in forces and stress. - Ultrasoft: QE says a larger value is often desirable, typically 8–12 times
ecutwfc. Treat that as typical guidance, not a fixed rule. - PAW: Four times
ecutwfcmay work, but the result depends on the augmentation-charge shape, so testing is mandatory.
Higher ecutrho may also be needed with gradient-corrected functionals, particularly for cells containing vacuum, or with pseudopotentials that lack nonlinear core correction. When the quantity is sensitive to density or its derivatives, test ecutrho separately while holding the other settings fixed. QE’s family-specific guidance and cautions are in the version 7.5 pw.x input reference; consult the matching reference for your installed release.
How dense should my k-point grid be?
There is no universal grid prescription in the QE references. Choose a uniform grid appropriate to the reciprocal geometry of your cell, then increase sampling and compare the target quantity. Consider the periodic directions individually: cell dimensions and reciprocal-cell geometry determine what a grid means, and changing to a supercell changes the appropriate grid dimensions.
The automatic-grid syntax is K_POINTS automatic, followed by nk1 nk2 nk3 sk1 sk2 sk3. The three nk values are grid dimensions; each offset value is 0 or 1. A 0 leaves that direction unshifted, while 1 shifts it by half a grid step. QE generates points according to the Monkhorst-Pack convention and, subject to symmetry settings, uses the irreducible Brillouin zone. The PW user guide’s input-data section describes automatic grids and symmetry handling.
For a convergence test, fix the offsets and increase the grid dimensions systematically, checking each periodic reciprocal direction as needed. Compare the same observable and tolerance used for your calculation. Do not assume a denser grid in one direction compensates for inadequate sampling in another.
- Metallic systems and occupation treatment can affect how much sampling is needed.
- Offsets matter: not every shifted grid has the full crystal symmetry required by tetrahedron integration.
- Symmetry settings affect which points are generated or retained; document settings such as
nosymwhen relevant.
Should I use the same k-point grid for SCF, DOS, and bands?
Not necessarily. Choose sampling for the calculation being performed and the output you need. The QE guide describes an SCF calculation followed by an NSCF calculation using the desired grid. For a density of states (DOS), it recommends a uniform automatically generated grid with tetrahedron occupations. A band structure instead uses an NSCF calculation at a fixed SCF potential and samples eigenvalues along a selected path (or another grid appropriate to the task). A path through selected points is not interchangeable with a uniform grid used for Brillouin-zone integration.
See the PW user guide’s electronic-structure section and the QE input-data FAQ for the documented calculation patterns.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11How should I run and report a reproducible convergence test?
- Fix and record the setup. Note the QE release, exchange-correlation setup, structure, pseudopotential filenames and types, and the quantity you will assess. Use documentation matching that release.
- Test
ecutwfc. Starting from a sensible pseudopotential-informed value, increase it in a controlled sequence. Hold other inputs fixed and compare the target observable against your declared tolerance. - Test
ecutrhowhere appropriate. Apply the pseudopotential-family guidance above, then vary it independently when needed. Do not infer its convergence from theecutwfctest. - Test the k-point mesh. With the selected cutoffs fixed, increase grid dimensions in the periodic reciprocal directions as needed, maintaining the chosen offset and other settings. Compare the same target observable.
- Check variable-cell optimizations. QE’s guide notes that plane waves and G-vectors use the starting cell during optimization and the final cell for the last step. A large difference between the last steps indicates the plane-wave basis may be far from converged; increase
ecutwfcand/orecutrhoand check again. See the PW user guide. - Document the evidence. Report the QE release, pseudopotentials, cutoffs, grid dimensions and offsets, relevant symmetry settings, convergence quantity, tolerance, and tested values with the changes observed.
FFT grid dimensions depend on the cutoff, cell, and FFT-library constraints. Different libraries or machines can produce different FFT dimensions and small energy differences, as the PW user guide explains. Recording computational details helps others interpret small differences when reproducing a calculation.
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