Molecular Structure Theory without the Born-Oppenheimer Approximation: Rotationless Vibrational States of LiH.

Area of Science:

• Quantum chemistry
• Molecular physics
• Computational chemistry

Background:

• The Born-Oppenheimer (BO) approximation is a cornerstone of molecular quantum mechanics, simplifying calculations by separating nuclear and electronic motion.
• Deviations from the BO approximation can be significant for light molecules like lithium hydride, impacting accuracy.
• Accurate theoretical treatments are crucial for understanding molecular structure and dynamics.

Purpose of the Study:

• To perform highly accurate, non-Born-Oppenheimer calculations for low-lying rotationless states of lithium hydride (LiH).
• To evaluate relativistic and quantum electrodynamics (QED) corrections to the molecular energies.
• To investigate the geometric structure of LiH in its ground and excited states.

Main Methods:

• Variational method utilizing explicitly correlated Gaussian basis functions for all six particles (two nuclei, four electrons).
• Analytic energy gradient optimization of nonlinear parameters in the wave function expansion.
• Perturbation theory for calculating leading-order relativistic and QED corrections.

Main Results:

• Obtained highly accurate non-BO energies and structural parameters for LiH states.
• Determined nucleus-nucleus correlation functions to analyze molecular geometry.
• Evaluated relativistic and QED corrections, providing insights into their contribution.

Conclusions:

• The non-BO approach provides a more accurate description of LiH compared to BO approximations.
• The study establishes new benchmarks for LiH properties, including relativistic and QED effects.
• Accurate computational methods are essential for precise molecular modeling and understanding quantum phenomena.