X2C Hamiltonian Models in ReSpect: Bridging Accuracy and Efficiency

Michal Repisky; Stanislav Komorovsky; Lukas Konecny; Marius Kadek; Torsha Moitra; Marc Joosten; Debora Misenkova; Rasmus Vikhamar-Sandberg; Martin Kaupp; Kenneth Ruud; Olga L. Malkina; Vladimir G. Malkin

arXiv · 2505.01088

X2C Hamiltonian Models in ReSpect: Bridging Accuracy and Efficiency

Michal Repisky·Stanislav Komorovsky·Lukas Konecny·Marius Kadek·Torsha Moitra·Marc Joosten·Debora Misenkova·Rasmus Vikhamar-Sandberg·Martin Kaupp·Kenneth Ruud·Olga L. Malkina·Vladimir G. Malkin

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Abstract

Since its inception, the ReSpect program has been evolving to provide powerful tools for simulating spectroscopic processes and exploring emerging research areas, all while incorporating relativistic effects, particularly spin-orbit interactions, in a fully variational manner. Recent developments have focused on exact two-component (X2C) Hamiltonian models that go beyond the standard one-electron X2C approach by incorporating two-electron picture-change corrections. This paper presents the theoretical foundations of two distinct atomic mean-field X2C models, amfX2C and extended eamfX2C, which offer computationally efficient and accurate alternatives to fully relativistic four-component methods. These models enable simulations of complex phenomena, such as time-resolved pump-probe spectroscopies and cavity-modified molecular properties, which would otherwise be computationally prohibitive. ReSpect continues to evolve, providing state-of-the-art quantum chemical methods and post-processing tools, all available free of charge through our website, www.respectprogram.org, to support researchers exploring relativistic effects across various scientific disciplines.

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Michal Repisky, Stanislav Komorovsky, Lukas Konecny, Marius Kadek, Torsha Moitra, Marc Joosten, Debora Misenkova, Rasmus Vikhamar-Sandberg, Martin Kaupp, Kenneth Ruud, Olga L. Malkina, Vladimir G. Malkin. 2025-05-02. X2C Hamiltonian Models in ReSpect: Bridging Accuracy and Efficiency. https://arxiv.org/abs/2505.01088

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