| 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.
