We present an ab initio many-body theory of optical activity in solids within the GW-BSE framework. Dielectric spatial dispersion is formulated in two complementary forms: exciton envelope modulation and sum-over-exciton-states expansion. Our application to α-quartz reveals that the envelope-modulated formulation captures the low-frequency region, whereas the sum-over-exciton-states formulation is essential to reproduce the correct full frequency dependence. Comparisons with the independent-particle approximation and simple local-field corrections further highlight the decisive role of excitonic many-body effects in shaping the spectral dispersion of optical activity in solids.
@article{arxiv.2604.05450,
title = {\textit{Ab initio} \textit{GW}-BSE theory of optical activity in $\alpha$-quartz},
author = {Xiaoming Wang and Yanfa Yan},
journal= {arXiv preprint arXiv:2604.05450},
year = {2026}
}