We present a first-principles study of the natural optical activity of the gyroelectric ${\mathrm{Pb}}_5{\mathrm{Ge}}_3{\mathrm{O}}_{11}$ crystal, explicitly accounting for spin-orbit coupling (SOC) effects. We derive a new analytical expression for the gyration coefficients within the recent framework of long-wavelength density-functional perturbation theory [Phys. Rev. Lett. 131, 086902 (2023)], which significantly improves computational efficiency by reducing the number of required response functions and includes spin-orbit coupling effects. We use this implementation to investigate the evolution of ${\mathrm{Pb}}_5{\mathrm{Ge}}_3{\mathrm{O}}_{11}^{\prime }\mathrm{s}$ optical rotation across the ferroelectric double-well, from the paraelectric $P\overline{6}$ phase to the ferroelectric $P3$ phase. Our results demonstrate that, in addition to the substantial renormalization of the double-well energy, spin-orbit coupling contributions play an equally crucial role in the natural optical activity, largely through purely electronic contributions, while SOC-induced structural relaxation effects are minor.

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• Dielectric properties
• Electric polarization
• Electronic structure
• Spin-orbit coupling
• First-principles calculations