Extending the reach of real-space real-time TDDFT: new developments in the Octopus code

Octopus is an open-source package implementing time-dependent density functional theory in a real-space, real-time formulation. Here we report on the developments since our previous contribution to the Special Topic Issue on Electronic Structure Software [J. Chem. Phys. 152, 124119 (2020)]. New exchange-correlation functionals extend the range of systems that can be treated, while advances in relativistic and all-electron methods open the door to core-electron spectroscopy. New capabilities for lattice dynamics - stress-tensor calculation, geometry optimization, support for machine-learned force fields, and phonon calculations - enable a fully ab initio description of light-induced lattice dynamics in real time. We also present developments that move beyond the dipole approximation, capturing light-matter interaction more faithfully. Improved methods for exciton dynamics complement a set of algorithmic advances - new propagation schemes, imaginary-time evolution, and a refactored grid handling more than a billion points - that together substantially improve performance and scalability. Combined with technical improvements such as AMD GPU support and a modernized build system, these developments consolidate Octopus as a versatile, high-performance platform for exploring electronic excitations, light-matter interaction, and nonequilibrium dynamics in molecules, nanostructures, and extended systems.