MCNOX: A code for computing and interpreting ultrafast nonlinear X-ray spectra of molecules at the multiconfigurational level

Core Hole Effect to Valence Excitations: Tracking and Visualizing the Same Excitation in XPS Shake-Up Satellites and UV Absorption Spectra

“…Natural transition orbitals (NTOs) provide a compact single-electron picture to describe multielectron transitions. NTOs have been widely used at the TDDFT , or multiconfigurational − levels to characterize valence and core excitations. Mathematically, they were generated from the singular value decomposition (SVD) for the single-particle transition density matrix boldP : boldP = boldU boldΛ boldV † Here, boldΛ is a diagonal matrix containing the singular values λ i , often referred to as the occupation numbers (ONs).…”

“…The summation of all s i values across all NTO pairs equals 1 (or 2 if considering a spin-summed transition density matrix). Both the singular values (λ i ) and their squares ( s i ) are used in the literature to characterize the amplitudes of the NTO pairs (and other varients are avialable , ), indicating an equivalent analysis of the dominant NTO pairs. In this work, we consistently adopted the eigenvalues s i to describe the different contributions (referred to as “contribution values” hereafter), as implmented in the Multiwfn 3.8 (dev) program …”

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra

“…Theoretically, ultrafast spectroscopic study of ultrafast chemical reactions follows a research paradigm of “electronic structure → molecular dynamics → signal” . The direct solution involves dynamics at various levels (such as ab initio, nondiabatic, quantum wavepacket), which is computationally expensive to reach the statistical average for relatively large systems or long dynamics. − This serves as the bottleneck of linking between experiment and simulation.…”

“…We will use the protocol of snapshots along the PES , to simulate the TXPS and TXAS spectra along the PT path, and to investigate the relationship between the proton transfer dynamics and X-ray spectra in such complex organic crystals. Within this framework, one obtains for instance, the TXAS signal S TXAS (ω, ζ) in terms of the reaction coordinate ζ , instead of the signal S TXAS (ω, τ) in terms of the time delay τ (ω denotes the photon energy) . At each snapshot, we map out the core ionization or excitation from the ground electronic state, providing computed maps of XPS or XAS that correspond to the proposed infrared (IR)-pump–X-ray-probe and the terahertz (THz)-pump–X-ray-probe spectroscopies.…”