Magic numbers, quantum delocalization, and orientational disordering in anionic hydrogen and deuterium clusters

Area of Science:

• Physical Chemistry
• Quantum Mechanics
• Computational Chemistry

Background:

• Anionic hydrogen clusters (H^-(H₂)_n) are model systems for studying quantum effects in matter.
• Previous studies suggested specific cluster sizes, like n=12, exhibit unique stability ('magic numbers').

Purpose of the Study:

• To investigate the structural and energetic properties of anionic hydrogen and deuterium clusters using advanced computational methods.
• To challenge the existence of 'magic numbers' in these systems and understand the role of quantum nuclear effects.

Main Methods:

• Application of the Diffusion Monte Carlo (DMC) method.
• Utilized a polarizable all-atom potential energy surface (PES) developed by Calvo and Yurtsever.
• Simulations covered hydrogen clusters (n=1-16, 32), deuterated analogs, and mixed H₂/D₂ clusters.

Main Results:

• Binding energy of hydrogen clusters shows a smooth dependence on size, refuting the 'magic number' claim for n=12.
• Low-energy structures exhibit icosahedral motifs, but ground state wavefunctions are delocalized due to strong nuclear quantum effects.
• Deuterium clusters show localized wavefunctions but disordered structures without size sensitivity.
• In mixed clusters, H₂ molecules are found farther from the central ion than D₂ molecules.

Conclusions:

• The concept of 'magic numbers' is not supported for these anionic hydrogen clusters.
• Nuclear quantum effects significantly influence cluster structure and dynamics, leading to orientational disorder.
• Isotopic differences in quantum behavior dictate molecular arrangement in mixed hydrogen-deuterium clusters.