Helium cluster ions: coherent charge sharing and the general trimerization trend.

Area of Science:

• * Computational Chemistry
• * Physical Chemistry
• * Quantum Chemistry

Background:

• * The coupled-monomers model treats molecular systems as interacting monomer networks.
• * It's a self-consistent density-matrix adaptation of Hückel Molecular Orbital (MO) theory.
• * Applied to inert X^± cluster ions, it models bonding variations using a bonding function β(χ).

Purpose of the Study:

• * To derive the bonding function for He⁺ using high-level ab initio data.
• * To illustrate general X^± bonding trends with the simplest inert species, helium.
• * To investigate the influence of local and remote interactions on cluster ion stability.

Main Methods:

• * Developed a bonding function β(χ) for He⁺ using ab initio data.
• * Employed two approaches: He₂⁺ potential and multicluster training points.
• * Tested models considering local bonds only versus local and remote pairwise interactions.

Main Results:

• * Remote forces in He⁺ (n ≥ 3) are destabilizing, contributing ~5% to covalent energy.
• * All model variations predicted similar structural outcomes, favoring trimer-ion formation.
• * Trimer-ion formation is a universal feature of X^± covalent networks without geometric constraints.

Conclusions:

• * The tendency for trimer-ion formation is driven by enthalpy-driven competition between charge sharing and localization.
• * This suggests many currently unknown trimer-ions may exist in cold environments like exoplanetary atmospheres.
• * The coupled-monomers model provides a framework for understanding bonding in inert cluster ions.