Abstract
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@, 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 L1 edges, and is capable of reproducing the splitting of the 1s1 (n + 1)p1 final states (n + 1 being the lowest virtual p orbital of the halides), it by and large fails to properly reproduce the 1s1 (n + 2)p1 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.
Highlights
X-ray absorption spectroscopy (XAS) is a powerful technique to probe the structural and electronic properties of molecules from an atomistic picture, since the absorbing photons in the X-ray energy range promote excitations of the core electrons to unoccupied or continuum states
Before investigating the outcome of the real-time propagation of the wavefunctions, it is instructive to analyze the differences between the different models: isolated atoms, embedding approaches (FDE and block-orthogonalized MME (BOMME)), and standard density functional theory (DFT) calculations
The frozen density embedding (FDE) results, on the other hand, underestimate the effect of the environment and show almost no difference to the free ion results, apart from the fact that a splitting of the peak, much less significant than that seen for BOMME or the supermolecule, is seen. This is a further indication that the FDE approach has not properly captured the perturbations to the virtual orbitals of chloride induced by the solvent. In this manuscript we have carried out an investigation, to the best of our knowledge for the first time, of the accuracy of fully quantum mechanical (QM/QM), DFT-based embedding approaches–namely, Frozen Density embedding (FDE) and block-orthogonalized ManbyMiller embedding (BOMME) approaches–in the description of core excitation spectra (XAS), obtained by the real-time propagation of the electron density, for model systems representing the hydration of halide ions, comprising the halide ions as active subsystems, and the eight water molecules in the first solvation shells as the environment
Summary
X-ray absorption spectroscopy (XAS) is a powerful technique to probe the structural and electronic properties of molecules from an atomistic picture, since the absorbing photons in the X-ray energy range promote excitations of the core electrons to unoccupied or continuum states. K edges correspond to 1s → (n + 1)p dipole transitions, n + 1, being the first virtual p level, implying that, in a simple picture, the edge position is a direct measure of molecular valence states, allowing us to monitor the effect of the local environment on a given atom within an analyte. The interpretation of such environmental interplay calls for electronic structure calculations that allow us to access the atomic and molecular energy levels. The theoretical modeling of XAS spectra implies the calculations of core-valence excitations
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