Optical Properties Calculations

Overview

This skill group covers the calculation of optical properties from first principles using Quantum ESPRESSO. The central quantity is the frequency-dependent dielectric function, from which all other optical properties (absorption coefficient, reflectivity, refractive index, optical conductivity) are derived.

The workflow always begins with a well-converged SCF calculation followed by QE's epsilon.x post-processing tool, which computes the dielectric function within the Random Phase Approximation (RPA) / independent-particle approximation.

MACE cannot compute optical properties. Optical response requires electronic wavefunctions and transition matrix elements, which are only available from DFT. MACE can be used to pre-relax the structure before a QE calculation.

Sub-Skills

Method Decision Guide

Need optical properties of a material?
  |
  +--> Step 1: Relax structure (MACE quick relax or QE vc-relax)
  |
  +--> Step 2: QE SCF with dense k-grid and many empty bands
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  +--> Step 3: epsilon.x to compute dielectric function
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  +--> Step 4: Post-process to desired property
         |
         +--> Dielectric function? --> dielectric-function/ skill
         |
         +--> Absorption, reflectivity, refractive index? --> absorption-spectrum/ skill
         |
         +--> Optical conductivity? --> optical-conductivity/ skill

Common Prerequisites

• Pseudopotentials: QE calculations require pseudopotential files. All sub-skills show how to download SSSP pseudopotentials automatically.
• Structure files: Start from a CIF, POSCAR, or build with pymatgen/ASE.
• Dense k-grid: Optical properties converge slowly with k-point density. Use at least 12x12x12 for bulk, denser for small-gap systems.
• Many empty bands: nbnd must be large enough to capture transitions up to the desired photon energy. Typically 2--4x the number of occupied bands.
• Python environment: pymatgen, ASE, numpy, scipy, matplotlib are pre-installed.
• QE executables: pw.x, epsilon.x, ph.x (for Born charges).

Important Notes

• QE epsilon.x computes the independent-particle (RPA) dielectric function. It does not include excitonic effects (use BSE/GW for that, not available in standard QE).
• PBE underestimates band gaps, which shifts optical absorption onset to lower energies. Use scissors correction (intersmear broadening can partially compensate but does not fix the gap).
• For metals, the intraband (Drude) contribution must be added separately -- see the optical-conductivity skill.
• All optical property sub-skills share the same SCF + epsilon.x foundation. Run SCF once, then derive multiple properties from the same epsilon.x output.