optical-properties - SKILL.md Agent Skill

name: optical-properties description: Optical Properties Calculations (6 sub-skills: absorption-spectrum, dielectric-function, joint-dos, optical-conductivity, slme, transition-dipole)

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

Sub-Skill	Directory	Description
Dielectric Function	`dielectric-function/`	Frequency-dependent dielectric function (real and imaginary parts), static dielectric constant, Born effective charges
Absorption Spectrum	`absorption-spectrum/`	Optical absorption coefficient, joint density of states, reflectivity, refractive index vs photon energy
Optical Conductivity	`optical-conductivity/`	Frequency-dependent optical conductivity from dielectric function, Drude model for metals, sum rules

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
  |
  +--> Step 3: epsilon.x to compute dielectric function
  |
  +--> 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.