quantum-espresso

Quantum ESPRESSO Workflow

Description

This skill covers end-to-end DFT calculations using Quantum ESPRESSO (QE), from input file construction and pseudopotential selection through SCF convergence, structural relaxation, band structure, and DOS workflows. QE is the dominant open-source plane-wave DFT code and the standard backend for many high-throughput databases, phonon codes (PHonon/DFPT), and ML potential pipelines. Invoke this skill when setting up or debugging a QE calculation, selecting pseudopotentials, parallelizing across HPC resources, or generating DFT reference data with an open-source code.

Domain Context

Quantum ESPRESSO solves the Kohn-Sham DFT equations using a plane-wave basis set on a regular real-space grid, with pseudopotentials or PAW datasets representing core electrons. The plane-wave expansion is controlled by ecutwfc (wavefunction cutoff, in Ry) and ecutrho (charge density cutoff, in Ry). Unlike VASP, QE uses open pseudopotential libraries (SSSP, PseudoDojo) rather than bundled PAW datasets, making pseudopotential choice an explicit user responsibility.

Key physical approximations and their consequences:

• Pseudopotential approximation: Core electrons are replaced by a smooth effective potential. The cutoff radius and the number of valence electrons depend on the pseudopotential. Harder (smaller cutoff radius) pseudopotentials require higher ecutwfc but are more transferable. Softer (larger cutoff radius) pseudopotentials are cheaper but may fail for highly compressed lattices or unusual oxidation states.
• Ultrasoft vs. norm-conserving vs. PAW: Ultrasoft pseudopotentials (USPP) achieve low ecutwfc at the cost of requiring a separately converged ecutrho (typically 8–12× ecutwfc). Norm-conserving (NC) pseudopotentials use ecutrho = 4 × ecutwfc and are required for linear-response properties and many GW/TDDFT applications. PAW reconstructs all-electron wavefunctions and gives accuracy close to all-electron at moderate cost.
• Periodic boundary conditions: QE assumes 3D periodicity. Molecules, surfaces, and wires in a box must be handled with sufficient vacuum and, for charged systems or polar slabs, Makov-Payne or Martyna-Tuckerman corrections.
• Density functional: The functional is selected via the pseudopotential — each PP is generated for a specific functional (PBE, PBEsol, LDA, etc.). Unlike VASP, the input_dft tag in &SYSTEM can override this, but should only be used with care and for compatible PP/functional pairs.