7.3 GW and BSE: a verified handoff to an external code
These are unexecuted teaching inputs and starting models. Original diagrams are schematics, not calculated results. Validate version-specific syntax, licensed or authorized data, numerical convergence and the scientific model before using this workflow.
7.3.1 Model, units and provenance
PW cutoffs and energies use Ry, common force output uses Ry/bohr, and pressure uses kbar. Geometry cards state their coordinate units. Different executables have distinct grammars and time-unit conventions.
Shared inputs, conventions and evidence
7.3.2 Unexecuted inputs and explicit deltas
Use the accompanying instructions to identify the parent calculation and placement of every delta; a snippet is not automatically a standalone input. Preserve all blank-line and file-provenance requirements.
7.3.2.1 Input block 1
Stage 1, QE SCF: converge density, geometry and pseudopotential cutoff.
Stage 2, QE NSCF: full compatible k sampling and many empty bands.
Save wavefunctions, eigenvalues and metadata.
Stage 3, converter: validate p2y support for this QE format/build.
Stage 4, Yambo setup: inspect imported lattice, electrons, bands and k points.
Stage 5, GW: converge screening and self-energy parameters independently.
Stage 6, BSE: add a separately converged electron-hole calculation if needed.
7.3.3 Worked investigation
7.3.3.1 Intuition and prerequisites
GW quasiparticle corrections and electron–hole BSE spectra are different calculations from semilocal DFT, hybrid DFT and independent-particle optics. This case defines a QE→Yambo route as an external-code dependency, not a claim that pw.x performs GW after a keyword change. Install a mutually compatible QE/Yambo pair and follow the converter's current support matrix. A conservative starting point is a small nonmagnetic semiconductor with a supported norm-conserving pseudopotential.
7.3.3.2 Original staged specification
See input block 1 above.
7.3.3.3 Outputs, checks and exercise
Validate electron count, lattice orientation, eigenvalues and band indexing after conversion before spending on GW. Build a convergence matrix: increasing empty bands alone does not converge the dielectric basis, and increasing dielectric cutoff alone does not converge screening sums. For 2D materials, vacuum and Coulomb truncation need dedicated tests. BSE requires its own k-grid and transition-space convergence; a converged quasiparticle gap is not a converged exciton binding energy.
Exercise: create a method ledger with four columns: Kohn–Sham gap, hybrid eigenvalue gap, GW quasiparticle gap and BSE optical onset. For each, state the defining approximation and required convergence evidence. Report a blocked conversion honestly; do not fill missing GW values with a scissor shift and relabel them as calculated GW.