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Modeling L-edge spectra at X-ray wavelengths requires consideration of spin–orbit splitting of the 2p orbitals. We introduce a low-cost tool to compute core-level spectra that combines a spin–orbit mean-field description of the Breit–Pauli Hamiltonian with nonrelativistic excited states computed using the semi-empirical density-functional theory configuration-interaction singles (DFT/CIS) method, within the state-interaction approach. Our version of DFT/CIS was introduced recently for K-edge spectra and includes a semi-empirical correction to the core orbital energies, significantly reducing ad hoc shifts that are typically required when time-dependent (TD-)DFT is applied to core-level excitations. In combination with the core/valence separation approximation and spin–orbit couplings, the DFT/CIS method affords semiquantitative L-edge spectra at CIS cost. Spin–orbit coupling has a qualitative effect on the spectra, as demonstrated for a variety of 3d transition metal systems and main-group compounds. The use of different active orbital spaces helps to facilitate spectral assignments. We find that spin–orbit splitting has a negligible effect on M-edge spectra for 3d transition metal species. 

High-harmonic generation (HHG) has been established as a powerful tool for studying structure and dynamics of quantum systems in gas and solid phases. To date, only a few studies have extended HHG spectroscopy to liquids, and much remains unresolved concerning the information that can be extracted from HHG spectra about the local liquid environment and the potential of HHG as a nonlinear probe of solvation dynamics. In this work, we investigate HHG in liquid binary solutions consisting of mixtures of aromatic benzene derivatives solvated in methanol. We observe evidence of a localized solvation structure that is imprinted on the harmonic spectra in the form of a strongly suppressed harmonic order, and an overall reduction of the total harmonic yield. We characterize this behavior as a function of laser parameters, concentration, and other halogenated benzene derivatives in methanol solution. Guided by theory, we interpret the results in terms of a localized solvation shell that is formed in specific solutions and acts like a local scattering barrier in the HHG process. This work demonstrates the potential of high-harmonic spectroscopy in liquids to extract detailed information about the structure and dynamics of solvation while expanding our understanding of the fundamental mechanism of HHG in systems with short-range order. 

ABSTRACT Fragment‐based quantum chemistry offers a means to circumvent the nonlinear computational scaling of conventional electronic structure calculations, by partitioning a large calculation into smaller subsystems then considering the many‐body interactions between them. Variants of this approach have been used to parameterize classical force fields and machine learning potentials, applications that benefit from interoperability between quantum chemistry codes. However, there is a dearth of software that provides interoperability yet is purpose‐built to handle the combinatorial complexity of fragment‐based calculations. To fill this void we introduce “Fragme∩t”, an open‐source software application that provides a tool for community validation of fragment‐based methods, a platform for developing new approximations, and a framework for analyzing many‐body interactions.Fragme∩tincludes algorithms for automatic fragment generation and structure modification, and for distance‐ and energy‐based screening of the requisite subsystems. Checkpointing, database management, and parallelization are handled internally and results are archived in a portable database. Interfaces to various quantum chemistry engines are easy to write and exist already for Q‐Chem, PySCF, xTB, Orca, CP2K, MRCC, Psi4, NWChem, GAMESS, and MOPAC. Applications reported here demonstrate parallel efficiencies around 96% on more than 1000 processors but also showcase that the code can handle large‐scale protein fragmentation using only workstation hardware, all with a codebase that is designed to be usable by non‐experts.Fragme∩tconforms to modern software engineering best practices and is built upon well established technologies including Python, SQLite, and Ray. The source code is available under the Apache 2.0 license. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsTheoretical and Physical Chemistry > ThermochemistrySoftware > Quantum Chemistry