Quantum fluctuation and vibrational dynamics of aqueous Cu and Ag clusters

Koizumi, Akihito*; Tachikawa, Masanori*; Shiga, Motoyuki   

The structures and infrared spectra of Ag(HO) ( = 1-4) and Cu (HO) are studied by -- MD, PIMD, and RPMD simulations. It is found that the anharmonicity due to nuclear quantum and thermal effects acts differently depending on the system as well as the type of vibrational mode. In the low-frequency region, the spectra become a broad band as the cluster size increases due to the complex mode couplings, which is qualitatively different from the harmonic spectra. In contrast, the HOH bending modes are systematically red-shifted by a small amount due to the anharmonicity. The OH stretching modes are found to be also red-shifted, but the amount of shift is more dependent on the system. Consequently, these effects cannot be described by unique scaling of harmonic frequencies.