Vibrational anharmonicity of small gold and silver clusters using the VSCF method

Area of Science:

• Computational Chemistry
• Materials Science
• Spectroscopy

Background:

• Understanding the vibrational spectra of small metal clusters is crucial for characterizing their properties.
• Anharmonicity, deviations from harmonic approximations, can significantly impact vibrational frequencies.
• Previous studies often relied on harmonic approximations, potentially overlooking anharmonic effects.

Purpose of the Study:

• To investigate and quantify anharmonicity in small neutral gold (Au2-Au10) and silver (Ag2-Ag5) clusters.
• To compare vibrational frequencies calculated with anharmonic methods against experimental data.
• To assess the influence of anharmonicity versus electronic structure methods and noble gas embedding on cluster vibrational properties.

Main Methods:

• Employed the vibrational self-consistent field (VSCF) method to account for anharmonicity.
• Utilized a vibrational configuration interaction (VSCF/VCI) approach for correlation-corrected calculations.
• Generated potential energy surfaces (PES) using periodic density functional theory (DFT) with plane-wave pseudopotentials.
• Implemented fast-VSCF and the Voter-Chen potential for efficient computation and reduction of pair-couplings.

Main Results:

• Gold clusters exhibit very small anharmonicity (max rms deviation ~1 cm-1), with some modes slightly exceeding 2 cm-1.
• Silver clusters show slightly larger anharmonicity compared to gold clusters.
• Discrepancies between calculated and experimental frequencies are more attributable to the electronic structure method's quality than to anharmonicity.
• Noble gas embedding influences vibrational properties more significantly than anharmonicity.

Conclusions:

• Anharmonic effects in small gold and silver clusters are generally minor.
• The accuracy of electronic structure calculations is a primary factor in matching experimental vibrational frequencies.
• Noble gas encapsulation is a more dominant factor affecting vibrational spectra than anharmonicity in these systems.