Part 1 · physics
Gold is not yellow because individual atoms simply “have a gold colour”. The colour emerges from the frequency-dependent reflection of a solid, and in gold that optical response is strongly shifted by relativistic electron dynamics.
From atomic number to colour
Gold has atomic number 79. Its inner electrons therefore move fast enough that a non-relativistic description becomes inadequate. Scalar-relativistic effects contract the 6s state and alter the 5d band, while spin–orbit coupling further splits and mixes the electronic states. Together these changes move important interband transitions relative to those in lighter silver.
The usual intuition is that blue light is absorbed more strongly while red and yellow are reflected. A quantitative colour requires more: a complex dielectric function, its conversion into reflectance, and integration against an illuminant and the CIE colour-matching functions.
The experimental anchor
This project uses the measured optical constants reported by Olmon and colleagues as its reference. With the official CIE 1931 2° observer and D65 illumination, those measurements reproduce the expected gold-like colour. That validates the colour-conversion chain, not the quantum algorithm.
Why involve a quantum computer?
DFT with spin–orbit coupling is an excellent way to construct the material basis. The difficulty grows when controlled electron interactions, real-time response and larger active spaces are added. A quantum processor represents the fermionic state in qubits, but the challenge then moves to active-space selection, circuit depth, noise and verification.
Sources and reproducibility
- Pyykkö & Desclaux (1979), relativistic effects in chemistry
- Christensen & Seraphin (1971), band structure and optical properties of gold
- Olmon et al. (2012), measured optical constants of gold
Project status: 8 August 2026. Numerical values come from the frozen local research artifacts; the original claim boundaries are deliberately preserved.