arXiv subjects

Muhammed A. Dada

Publications and source records attributed to Muhammed A. Dada.

3 recordsLinked to original sources

arXiv2026

Toward Accurate RIXS Spectra at Heavy Element Edges: A Relativistic Four-Component and Exact Two-Component TDDFT Approach

We present a relativistic time-dependent density functional theory (TDDFT) approach for the simulation of resonant inelastic X-ray scattering (RIXS) spectra, based on both a full four-component (4c) Dirac-Coulomb Hamiltonian and a modern atomic mean-field exact two-component (amfX2C) Hamiltonian model. The approach builds on the pseudo-wavefunction formalism and a core-valence separation scheme, enabling the efficient evaluation of couplings between two manifolds of excited states relative to a common ground state, as required for solving the Kramers-Heisenberg equation for RIXS. The relativistic formulation provides a variational description of scalar and spin-orbit relativistic effects, which are essential for accurately describing inner-shell excitations involved in RIXS processes. Its transformation to the 2c regime via the amfX2C Hamiltonian significantly reduces the computational cost while offering 4c-quality results by accounting for two-electron and exchange-correlation picture-change effects arising from the X2C transformation. In addition to two-dimensional RIXS maps, the methodology enables the direct evaluation of high-energy-resolution fluorescence detection (HERFD) and resonant X-ray emission spectra (RXES). Applications to 2p3d and 3d4f RIXS maps of selected ruthenium and uranium complexes demonstrate that the amfX2C approach reproduces reference 4c results and experimental spectra with high accuracy, capturing all key spectral features and providing reliable peak assignments.

physics.chem-ph↗

arXiv2025

Quantifying the impact of the Tamm-Dancoff approximation on the computed spectra of transition-metal systems

The Tamm-Dancoff Approximation (TDA) offers a computationally efficient alternative to full linear-response Time-Dependent Density Functional Theory (TDDFT) for calculating electronic excited states, particularly in large molecular systems. By neglecting the coupling between excitation and de-excitation channels, TDA simplifies the TDDFT response equations into a Hermitian form. This not only reduces computational cost but also eliminates numerical instabilities that can arise in the full non-Hermitian formalism. While TDA has been widely explored for valence excitations, its reliability for transition metal complexes and core-level spectroscopies remains largely untested. In this work, we address this gap by systematically comparing TDA and full TDDFT results for a series of transition metal species, focusing on absorption spectra across the UV-Vis, metal K-edges, and L-edges. Our results show that, for core-level excitations, TDA yields excitation energies and oscillator strengths nearly indistinguishable from those obtained with full TDDFT. This agreement is attributed to the negligible contribution of de-excitation amplitudes at high excitation energies, indicating that the omitted coupling terms play an insignificant role in these spectral regimes. These findings validate the accuracy and robustness of TDA in core-level spectra simulations and support its broader application in studies involving transition metal systems.

physics.chem-ph↗

arXiv2025

Fast simulation of soft x-ray near-edge spectra using a relativistic state-interaction approach: Application to closed-shell transition metal complexes

Spectroscopic techniques based on core-level excitations provide powerful tools for probing molecular and electronic structures with high spatial resolution. However, accurately calculating spectral features at the L or M edges is challenging due to the significant influence of spin-orbit and multiplet effects. While scalar-relativistic effects can be incorporated at minimal computational cost, accounting for spin-orbit interactions requires more complex computational frameworks. In this work, we develop and apply the state-interaction approach, incorporating relativistic effects using the ZORA-Kohn-Sham Hamiltonian, to simulate near-edge soft X-ray absorption spectra for closed-shell transition metal complexes. The computed spin-orbit splittings closely match those obtained from more rigorous methods. This approach provides a practical and cost-effective alternative to more rigorous two-component methods, making it particularly valuable for large-scale calculations and applications such as resonant inelastic X-ray scattering simulations, where capturing a large number of excited states is essential.

physics.chem-ph↗