X2C Hamiltonian Models in ReSpect: Bridging Accuracy and Efficiency

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Relativistic Quantum Mechanics

Background:

• The ReSpect program has historically provided tools for spectroscopic simulations, including relativistic effects.
• Previous advancements focused on one-electron exact two-component (X2C) Hamiltonian models.
• Incorporating two-electron picture-change corrections is crucial for higher accuracy in relativistic calculations.

Purpose of the Study:

• To introduce and theoretically establish two novel atomic mean-field X2C (amfX2C) models.
• To present extended eamfX2C as a computationally efficient and accurate alternative to four-component relativistic methods.
• To enable the simulation of complex spectroscopic phenomena previously limited by computational cost.

Main Methods:

• Development of theoretical foundations for two distinct atomic mean-field X2C models.
• Implementation of two-electron picture-change corrections within the X2C framework.
• Focus on variational treatment of relativistic effects, particularly spin-orbit interactions.

Main Results:

• Introduction of amfX2C and extended eamfX2C models.
• Demonstration of computational efficiency and accuracy compared to four-component methods.
• Enabling simulations of advanced spectroscopies like time-resolved pump-probe and cavity-modified molecular properties.

Conclusions:

• The new amfX2C models offer significant improvements for relativistic calculations in spectroscopy.
• ReSpect continues to evolve, providing state-of-the-art, free computational tools for researchers.
• These advancements facilitate the exploration of relativistic effects in diverse scientific disciplines.