Quantum ESPRESSO Reference

Q: What is the difference between SCF, relax, and vc-relax calculations?

SCF (self-consistent field) computes the electronic ground state for fixed atomic positions and cell parameters. Relax optimizes atomic positions while keeping the cell fixed, using BFGS or damped dynamics. vc-relax (variable-cell relaxation) optimizes both atomic positions and cell parameters simultaneously, essential for finding equilibrium lattice constants or studying pressure effects. Each mode has specific convergence parameters detailed in this reference.

Q: How do I set up a k-point mesh for my calculation?

For SCF calculations, use automatic Monkhorst-Pack grids (K_POINTS automatic) with density inversely proportional to cell dimensions. A common starting point is 8x8x8 for cubic cells, reducing for larger cells. For band structure calculations, use K_POINTS crystal_b with high-symmetry points connected by a specified number of interpolation points. For metals, denser k-grids and appropriate smearing are required. The reference includes grid recommendations for different system types.

Q: Which exchange-correlation functional should I use?

PBE (Perdew-Burke-Ernzerhof) is the most widely used GGA functional, suitable for most solid-state calculations. PBEsol improves lattice constants for solids. LDA (local density approximation) is simpler but overbinds. For band gaps, consider hybrid functionals (HSE06) or DFT+U for transition metal oxides. The choice also depends on the available pseudopotentials. This reference covers functional specification syntax and common parameter values.

Q: How do I perform DFT+U calculations in Quantum ESPRESSO?

DFT+U corrections are enabled by setting lda_plus_u = .true. in the &SYSTEM namelist, then specifying Hubbard_U values for each atomic species (in eV). Typical U values range from 2-6 eV for transition metal d-orbitals. The newer DFT+U+V implementation uses Hubbard_V for inter-site corrections. The reference includes parameter syntax, recommended U values for common materials, and self-consistent U calculation approaches (HP code).

Q: What smearing method and degauss value should I use for metals?

For metallic systems, Marzari-Vanderbilt cold smearing (smearing = "mv") is generally recommended as it provides accurate forces. Methfessel-Paxton first-order (smearing = "mp") is also common. The degauss value (smearing width) typically ranges from 0.01-0.05 Ry; start with 0.02 Ry and check convergence. For insulators and semiconductors, use fixed occupations (occupations = "fixed") without smearing. The reference details all available smearing options.

Q: How do I compute a band structure with Quantum ESPRESSO?

Band structure calculation requires three steps: (1) Run an SCF calculation to obtain the ground-state charge density, (2) Run an nscf calculation with a dense k-point path along high-symmetry directions using the saved charge density, (3) Extract and plot bands using bands.x post-processing. The reference provides the complete workflow with example input files for each step, including high-symmetry k-point paths for common crystal structures (FCC, BCC, HCP).

Q: What post-processing tools are available in Quantum ESPRESSO?

Key post-processing tools include: pp.x for extracting charge density, electrostatic potential, and wavefunction data on real-space grids; dos.x for computing the electronic density of states; projwfc.x for projected density of states (PDOS) onto atomic orbitals; bands.x for band structure extraction; epsilon.x for optical properties; and ph.x for phonon calculations. Each tool reads the output of a preceding pw.x calculation. The reference covers input syntax for all major post-processing tools.

Free reference guide: Quantum ESPRESSO Reference

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About Quantum ESPRESSO Reference

The Quantum ESPRESSO Reference is a searchable guide to the input parameters and workflows of Quantum ESPRESSO, the open-source suite for electronic structure calculations based on density functional theory (DFT). It covers pw.x (plane-wave self-consistent field) calculations including SCF, relax, vc-relax, nscf, and bands modes, with detailed parameter descriptions for energy cutoffs (ecutwfc, ecutrho), k-point grids, smearing methods, and pseudopotential selection.

This reference is organized into five categories: pw.x Basics (calculation types, cutoff energies, convergence thresholds), pw.x Input (crystal structure definition, atomic positions, cell parameters, k-point meshes), pw.x Electronic (exchange-correlation functionals, smearing, mixing schemes, DFT+U corrections), pw.x Relaxation (BFGS optimization, force/energy convergence criteria, variable-cell relaxation), and Post-processing (pp.x charge density visualization, dos.x density of states, bands.x band structure extraction, projwfc.x projected DOS).

Designed for computational materials scientists, condensed matter physicists, and chemistry researchers, this tool provides instant access to Quantum ESPRESSO parameter syntax with recommended values and convergence testing guidance. Whether you are setting up an SCF calculation for a bulk crystal, relaxing atomic positions in a slab model, computing phonon dispersions with ph.x, or generating charge density plots with pp.x, this reference delivers the parameter details and example input blocks you need. All content runs entirely in your browser.

Key Features

Complete pw.x calculation type reference covering SCF, relax, vc-relax, nscf, and bands modes with example inputs
Energy cutoff parameter guide for ecutwfc and ecutrho with pseudopotential-dependent recommendations (PAW/USPP/NC)
K-point mesh configuration reference including automatic Monkhorst-Pack grids and explicit k-path definitions for band structures
Exchange-correlation functional reference covering LDA, GGA-PBE, PBEsol, hybrid functionals, and DFT+U/V corrections
Structural relaxation parameters for BFGS optimization with force convergence (forc_conv_thr) and energy convergence criteria
Post-processing tool reference for pp.x (charge density, potential), dos.x (density of states), and projwfc.x (projected DOS)
Smearing method guide covering Gaussian, Methfessel-Paxton, Marzari-Vanderbilt cold smearing, and Fermi-Dirac with degauss values
Searchable across all five categories with instant filtering, Python/Fortran-style input syntax, and dark mode support

Frequently Asked Questions

How do I choose ecutwfc and ecutrho cutoff values?

The wavefunction cutoff (ecutwfc) depends on the pseudopotential type: PAW and ultrasoft pseudopotentials (USPP) typically need 30-80 Ry, while norm-conserving (NC) pseudopotentials require 60-100+ Ry. The charge density cutoff (ecutrho) should be 4x ecutwfc for NC pseudopotentials and 8-12x for PAW/USPP. Always perform a convergence test by increasing cutoffs until total energy changes are below your target threshold (typically < 1 mRy/atom).

What is the difference between SCF, relax, and vc-relax calculations?

SCF (self-consistent field) computes the electronic ground state for fixed atomic positions and cell parameters. Relax optimizes atomic positions while keeping the cell fixed, using BFGS or damped dynamics. vc-relax (variable-cell relaxation) optimizes both atomic positions and cell parameters simultaneously, essential for finding equilibrium lattice constants or studying pressure effects. Each mode has specific convergence parameters detailed in this reference.

How do I set up a k-point mesh for my calculation?

For SCF calculations, use automatic Monkhorst-Pack grids (K_POINTS automatic) with density inversely proportional to cell dimensions. A common starting point is 8x8x8 for cubic cells, reducing for larger cells. For band structure calculations, use K_POINTS crystal_b with high-symmetry points connected by a specified number of interpolation points. For metals, denser k-grids and appropriate smearing are required. The reference includes grid recommendations for different system types.

Which exchange-correlation functional should I use?

PBE (Perdew-Burke-Ernzerhof) is the most widely used GGA functional, suitable for most solid-state calculations. PBEsol improves lattice constants for solids. LDA (local density approximation) is simpler but overbinds. For band gaps, consider hybrid functionals (HSE06) or DFT+U for transition metal oxides. The choice also depends on the available pseudopotentials. This reference covers functional specification syntax and common parameter values.

How do I perform DFT+U calculations in Quantum ESPRESSO?

DFT+U corrections are enabled by setting lda_plus_u = .true. in the &SYSTEM namelist, then specifying Hubbard_U values for each atomic species (in eV). Typical U values range from 2-6 eV for transition metal d-orbitals. The newer DFT+U+V implementation uses Hubbard_V for inter-site corrections. The reference includes parameter syntax, recommended U values for common materials, and self-consistent U calculation approaches (HP code).

What smearing method and degauss value should I use for metals?

For metallic systems, Marzari-Vanderbilt cold smearing (smearing = "mv") is generally recommended as it provides accurate forces. Methfessel-Paxton first-order (smearing = "mp") is also common. The degauss value (smearing width) typically ranges from 0.01-0.05 Ry; start with 0.02 Ry and check convergence. For insulators and semiconductors, use fixed occupations (occupations = "fixed") without smearing. The reference details all available smearing options.

How do I compute a band structure with Quantum ESPRESSO?

Band structure calculation requires three steps: (1) Run an SCF calculation to obtain the ground-state charge density, (2) Run an nscf calculation with a dense k-point path along high-symmetry directions using the saved charge density, (3) Extract and plot bands using bands.x post-processing. The reference provides the complete workflow with example input files for each step, including high-symmetry k-point paths for common crystal structures (FCC, BCC, HCP).

What post-processing tools are available in Quantum ESPRESSO?

Key post-processing tools include: pp.x for extracting charge density, electrostatic potential, and wavefunction data on real-space grids; dos.x for computing the electronic density of states; projwfc.x for projected density of states (PDOS) onto atomic orbitals; bands.x for band structure extraction; epsilon.x for optical properties; and ph.x for phonon calculations. Each tool reads the output of a preceding pw.x calculation. The reference covers input syntax for all major post-processing tools.