Environment Effects on X-Ray Absorption Spectra With Quantum Embedded Real-Time Time-Dependent Density Functional Theory Approaches

In this work we implement the real-time time-dependent block-orthogonalized Manby-Miller embedding (rt-BOMME) approach alongside our previously developed real-time frozen density embedding time-dependent density functional theory (rt-TDDFT-in-DFT FDE) code, and investigate these methods' performance in reproducing X-ray absorption spectra (XAS) obtained with standard rt-TDDFT simulations, for model systems comprised of solvated fluoride and chloride ions ([X@ ( H2O)(8)](-) , X = F, Cl). We observe that for ground-state quantities such as core orbital energies, the BOMME approach shows significantly better agreement with supermolecular results than FDE for the strongly interacting fluoride system, while for chloride the two embedding approaches show more similar results. For the excited states, we see that while FDE (constrained not to have the environment densities relaxed in the ground state) is in good agreement with the reference calculations for the region around the K and L-1 edges, and is capable of reproducing the splitting of the 1s(1) (n + 1)p(1) final states (n + 1 being the lowest virtual p orbital of the halides), it by and large fails to properly reproduce the 1s(1) (n + 2)p(1) states and misses the electronic states arising from excitation to orbitals with important contributions from the solvent. The BOMME results, on the other hand, provide a faithful qualitative representation of the spectra in all energy regions considered, though its intrinsic approximation of employing a lower-accuracy exchange-correlation functional for the environment induces non-negligible shifts in peak positions for the excitations from the halide to the environment. Our results thus confirm that QM/QM embedding approaches are viable alternatives to standard real-time simulations of X-ray absorption spectra of species in complex or confined environments.

Automated summary

In this work, the performance of real-time time-dependent block-orthogonalized Manby-Miller embedding (rt-BOMME) approach is investigated in reproducing X-ray absorption spectra (XAS) obtained with standard real-time frozen density embedding time-dependent density functional theory (rt-TDDFT-in-DFT FDE) simulations. Model systems of solvated fluoride and chloride ions ([X@ ( H2O)(8)](-) , X = F, Cl) are considered. The results show that the BOMME approach provides significantly better agreement with supermolecular results in ground-state quantities compared to FDE for the strongly interacting fluoride system, while for chloride the two methods show similar results. For excited states, the BOMME approach provides a faithful qualitative representation of the spectra in all energy regions considered, but it induces non-negligible shifts in peak positions for the excitations from the halide to the environment due to its lower-accuracy exchange-correlation functional. The study concludes that QM/QM embedding approaches are viable alternatives for real-time simulations of X-ray absorption spectra of species in complex or confined environments.