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

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Simulating X-ray absorption spectra (XAS) for solvated species is computationally demanding.
• Quantum mechanics/quantum mechanics (QM/QM) embedding methods offer a potential solution by treating different parts of the system at varying levels of theory.
• Real-time time-dependent density functional theory (rt-TDDFT) is a standard method for XAS calculations.

Purpose of the Study:

• To implement and assess the real-time block-orthogonalized Manby-Miller embedding (rt-BOMME) approach alongside a real-time frozen density embedding (rt-TDDFT-in-DFT FDE) code.
• To compare the performance of rt-BOMME and FDE in reproducing X-ray absorption spectra (XAS) against standard rt-TDDFT simulations.
• To investigate the accuracy of these QM/QM embedding methods for solvated fluoride and chloride ions.

Main Methods:

• Implementation of the real-time time-dependent block-orthogonalized Manby-Miller embedding (rt-BOMME) method.
• Utilized a previously developed real-time frozen density embedding time-dependent density functional theory (rt-TDDFT-in-DFT FDE) code.
• Performed simulations on model systems of solvated fluoride and chloride ions, comparing results to supermolecular rt-TDDFT.

Main Results:

• For ground-state core orbital energies, rt-BOMME showed better agreement with supermolecular results than FDE for fluoride ions.
• FDE accurately reproduced XAS features near the K and L1 edges but struggled with higher energy states and solvent-influenced excitations.
• rt-BOMME provided a good qualitative representation of the entire XAS spectrum, despite minor peak shifts due to approximations in the environment's functional.

Conclusions:

• QM/QM embedding approaches are viable alternatives for simulating XAS of species in complex or confined environments.
• rt-BOMME offers a more faithful qualitative reproduction of XAS spectra compared to FDE for the studied systems.
• Further development of QM/QM embedding methods can improve the accuracy of XAS simulations for challenging chemical systems.