An ab initio multireference perturbation theory study on the manganese dimer.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• The electronic structure and bonding of diatomic molecules are fundamental to understanding chemical interactions.
• Manganese dimer (Mn2) has been experimentally identified as a van der Waals molecule, but its precise electronic states require detailed theoretical investigation.

Purpose of the Study:

• To calculate the potential energy curves for the ground and excited states of the manganese dimer (Mn2).
• To elucidate the electronic structure and bonding characteristics of Mn2, particularly its ground state properties.
• To compare theoretical predictions with existing experimental data and other computational methods.

Main Methods:

• Employed second-order n-electron valence state perturbation theory (NEVPT2).
• Utilized a complete active space self-consistent field (CASSCF) reference wave function.
• Calculations covered a wide range of internuclear distances for accurate potential energy curve determination.

Main Results:

• The ground state of Mn2 is predicted to be a singlet (Σg+) with a large equilibrium internuclear distance (Re ≈ 3.7-3.8 Å).
• The ground state exhibits a low dissociation energy (De ≈ 0.07-0.08 eV) and a small harmonic frequency (ωe ≈ 43 cm⁻¹), confirming its van der Waals nature.
• A low-lying excited state, (11)Πu, was identified with distinct properties (Re = 2.50 Å, De = 1.35 eV, ωe = 246 cm⁻¹), contrasting with density functional theory predictions.

Conclusions:

• The theoretical calculations strongly support the experimental classification of Mn2 as a van der Waals molecule.
• The study provides accurate quantitative data for the ground and key excited states of Mn2.
• The findings highlight the importance of advanced quantum chemical methods for describing weakly bonded systems.