Full Minimal Coupling All-Electron Real-Time TDDFT for X-Ray-Matter Interactions

Standard real-time time-dependent density functional theory (RT-TDDFT) couples light to matter through a spatially uniform (dipole) vector potential, an approximation that breaks down in the X-ray regime, where the photon wavelength approaches interatomic scales. We present an all-electron, numeric-atom-centered-orbital implementation of full minimal coupling in FHI-aims that retains the spatial structure of the vector potential in both the paramagnetic and diamagnetic couplings, implements a reciprocal-space-resolved current diagnostic, and extends the propagation to periodic systems through a perturbative photon-momentum sideband coupling that recovers the density response at the photon wavevector. Four benchmarks ranging in energy from 100 eV to 5 keV validate the implementation and exercise its reach. The retained photon momentum proves physically consequential throughout, from measurable beyond dipole corrections to the enabling of channels forbidden in the dipole limit. On an all-electron footing, the implementation lays the groundwork for a general ab initio treatment of X-ray light-matter interaction across molecules, surfaces, and solids.